As an engineer, the idea of a power meter on my bike is an extremely attractive one. The basic concept of a unambiguous, objective method to monitor performance that can cut through all of the outside influences (fatigue, wind, drafting, hills, road surfaces, etc.) is a powerful training tool. Unfortunately, the cost of these sensors is often difficult to justify unless you are a high level athlete (which I'm most certainly not). Most of the solutions out there cost upwards of $2000, and at this juncture there are other things that would likely be more beneficial for me at this point (eg a dedicated TT bike).
The Saris PowerTap is probably the closest direct measurement power meter on the market right now, and while it can be had for less than $1000 it needs to be installed into a wheel (or purchased with one). Unless you are planning on upgrading wheelsets, that means that the effective price of adding one of those units is a good deal higher. Further, eventually I'd like to have a separate wheelset for racing vs. training, and with the PowerTap solution that means buying more than one power meter.
Polar's CS600X power meter falls into a lower price bracket and, as it is external to the powertrain, doesn't need any modifications to the bike. Additionally, Polar does offer a lot of interesting information that other vendors don't - such as left-right balance and indications of the roundness of the stroke. How accurate these values are I'm not sure, but for someone still relatively new to the sport they could be quite useful for improving technique. Finally, Polar's heart rate straps are much better than the Garmin ones used on all of the ANT+Sport units that work with the aftermarket power meters, so that's a big plus on their side (Suunto has competitive HRMs, but they appear to use a different variant of the ANT protocol that is incompatible with other products).
Unfortunately, the more that I read about it the more concerned I am about its accuracy. It measures power indirectly, by picking up chain speed and vibration and working back to the power number. This is an ingenious idea in-and-of-itself, unfortunately it is susceptible to noise introduced from road vibrations and varying distances between the sensor and chain as you change gears. Due to this, it is very sensitive to how it is installed on the bike and as I have yet to find a local store with any experience with the unit I'm a bit uncomfortable with the concept. If I could find a store that has installed a few dozen of these I might think about it, but unfortunately that doesn't appear to be an option around here.
The most promising solution I've seen until recently was the iBike line, which doesn't so much measure power output but measures the opposing forces and figures out how much power would be required to explain the current velocity/acceleration. Aside from providing power readings, this solution has the benefits of also recording exactly where that power is going - having airspeed measurements and a proper inclinometer (ie not just dividing elevation change by distance covered) could be invaluable for analyzing a ride after-the-fact. Further, when paired with another ANT+ power meter it can provide real time readings on your aerodynamic drag coefficient (great for fine-tuning body positions on the bike).
The problem with this unit is that its design has a lot of rough edges (kludgy non-customizable UI, wireless radios not build into the head unit and insufficient battery life) largely due to them using the same hardware platform for their $199 and $799 products. Further, its heavy reliance on the airspeed sensor means that it can't provide accurate power readings when riding in heavy rain (as the port gets plugged with water). Still, it's an excellent concept and its shortcomings should be easy for Velocomp to address in any future iterations (ie add at least two bitmapped lines to the display for proper alphanumeric menus, add the capacity to customize the data fields shown on screen, put the wireless bits in the head unit rather than the mount and move to a bigger battery (eg a CR123 or even rechargeable cellphone batteries)).
Looking over the coverage of Interbike last week, however, a company called MetriGear caught my attention with a new power meter called Vector. Rather than measuring at the crank (SRM and Quarq) or hub (PowerTap), they put a small sensor in each of the pedal spindles. As such, they are trivial to install and don't require you to replace any major components of the bike. Further, as each foot has its own sensor this technology has the potential to be able to provide more detailed information about how the rider is delivering that power. This means they could provide the left-right balance and stroke quality readings like Polar are providing, but with a lot more detail as these values could be directly measured rather than extrapolated (although this will be dependent on a head unit that can capture and record this extra data).
Adding to this, like the PowerTap it should be very easy to move one set of sensors over to another bike as necessary. As such, if I do eventually get a dedicated TT bike to go along with the road bike, this would make it feasible to have a single power meter that could be used by both. The crank-based designs are a pretty complex ordeal to swap over (especially for someone like myself with limited experience changing out parts), and while the iBike is pretty simple the fact that they put the wireless bits in the mount means that it's an expensive proposition.
The big news, however, is that they are talking about comming in at a price below $1000 including the pedals. That's still not innexpensive by any means, however it does fall into the range where I might consider picking up a power meter. The price doesn't include a head unit (they don't appear to have any plans to make one), but as they are ANT+ compatible they will work with a number of offerings already on the market. The question remains whether the aftermarket head units (Garmin's 310xt, 500 and 705 and the iBike Pro and Aero products) will be able to understand the unique data these sensors can capture as the current firmware seems to simply understand basic power data.
At this point I'm not really in a position to buy one of these (not that it matters as they won't be available until Q1 2010), however I am in the process of buying a new cyclocomputer (as I've written a few times here) and this development is enough for me to strongly re-focus on products that will be compatible with this unit. The Polar CS600X (likely without power) was strongly leading the pack before this announcement, but now I'm seriously reconsidering the Edge 705 or potentially waiting for the Edge 500 as they will both be compatible with the Vector. I've always preferred much of the hardware design of the 705 over the Polar, but I'm still a bit concerned about its perpetually crappy firmware (stability issues, horribly inaccurate caloric readings, unreliable elevation recording, etc.). The Edge 500 may address those issues as it's a much simpler design, but it gives up a lot of what made the 705 attractive (street level mapping, ability to upgrade memory, etc.).
The iBike iAero is also an option, although that is a more costly option and, as above, I have some reservations about their design. I was hoping that they'd announce a fourth generation design of their product this year, but it doesn't look like that is going to happen. They have made great strides on their firmware over the last little while, and appear to be by far the best at addressing user concerns with periodic updates. Unfortunately, firmware upgrades can't address the fundamental shortcomings of the underlying hardware platform so it remains an unlikely choice despite its compelling feature set.
Either way, it seems that every time I'm about ready to settle down for one choice or the other a wrench like this gets thrown into my plans ;) Garmin is good at making excellent hardware, but they aren't very good at the firmware side. iBike, on the other hand, is great at squeezing every last bit of capability out of their devices with firmware updates, but their hardware is limited due to their insistence on a single platform for such a wide range of price points. Polar was the happy middle ground (not the most feature packed, but exquisitely engineered hardware and software), but their lack of support for the ANT+ protocol is a big limitation with the advent of power meters such as the Vector.
With the Toronto bike show comming up in a couple of weeks, I'll probably go and see if there are any decent deals on these things. Barring that, I'll just have to mill over the choices a bit more ;)