i have owned and installed on two different bikes (in chronological order): two polars (720i), one ergomo pro, and two power taps.
1). Since I owned the polar 720i heart rate monitory & cadence/RPM device, i decided for the money, it was the cheapest way to begin measuring power. i generally was happy but sometimes found in certain cassette/ring combinations, i might get slightly unusual readings. in the big scheme of things, I could live with this because i am a recreational rider. i found that the installation of the units were pretty straightforward, and polar now has a great installation video on their website. There are some oddities about the unit such as needing to know the weight & length of the chain, but once you set the unit up, you forget about this sorta thing. i used them on both bikes for about 2 years.
2). after deciding i wanted to increase my investment in power equipment,. i thought there might be something better than the polars. plus i wanted to remove the polar power-reading device on my rear chainstay. I paid about $1,600 for the ergomo on the STRONG recommendation of my local bike shop. they told me that EVERYONE was raving about the ergomo and it was better to go that route than a power tap. after a few weeks of using the ergomo, my initial impression was very positive. i liked the huge display and its flexible way of changing what is displayed on the screen (very customizable). i also liked the altimeter and the % gradient reading it gave me. i was curious that the power measurements were very close to my old polar unit on the same bike. i began to believe that the ergomo generally confirmed that the polar was reading power reasonably accurately. i rode this bike for about 9 months with the ergomo. i strapped the polar HRM to my tandem so I can track my rides on that bike. i don't have the power unit connected (not compatible with a tandem).
3). Just to get some variability, i then put a powertap on my other bike. i paid about $900 on Ebay. i found it measured about the same power as the polar did. i thought the polar had a far more flexible display. i disliked not being able to change rear wheels (the rear hub becomes IS the power reading unit), but i don't change rear wheels too often so it was not a big deal. I liked the ergomo display much better than the power tap display. i was not dissatisfied with the power tap, though. it seemed very consistent. i rode this bike for about 6 months.
4). after not riding my Ergomo bike for approx. 6 months, i picked it back up again. unfortunately the ergomo unit immediatly began behaving strangely (wild fluctuations of power, blank display, etc). resetting the K factor did no good, resetting the unit did no good. so i bought another powertap on Ebay (about $650) and installed it on that bike. I took the ergomo back to my local bike shop for repair. They found the ergomo bottom bracket sensors were defective and they had to ship the unit back to Germany since the US distributor stopped working with Ergomo. that was approx. 2 months ago and i have not received a new unit yet. Needless to say, it is not good to pay $1,600 for something and have it break down and require repair in a foreign country with no end in sight.
5). a problem i have with one of my powertap units is the computer will not zero the watts (this is a way of periodically resetting the torque and wattage settings). the problem started when i accidentally put one computer on the other bike for a trainer ride. i noticed my wattage seemed about 10 - 15 watts too once I moved it back to my old bike. when i attempted to zero the torque/watts, i would go no lower than 11 watts. i haven't tried to figure this one out yet, but i know that i am running 11 watts to high on my 2nd bike. i am sure there is a simple fix, but i have not found it yet.
bottom line: i would NOT recommend the ergomo due to aforementioned problem, i would recommend the polar (it is a decent unit for the money), and i would recommend power tap (it works & is consistent). I suppose if I get my ergomo back (eventually) and it works flawlessly for months, i may change my mind. once it does come back from Germany, i DO plan to ride with both a powertap & ergomo on the same bike to compare how they read.
by the way, i have used all three power meters on & off a trainer, and found no anomolies. All three worked as well on a trainer as on the road. my two bikes are: Specialized Roubaix Pro (carbon) and a Cannondale CADD 7 (aluminum)
the beauty of the polar is that for about $400 - $500, you can get into measuring power. i think the next step up if you can afford it ($600 - $1,000 depending on new/used & wired/wireless) it is powertap. Of course, if you want to go all out, spend $2,500 on an SRM unit.
i would encourage you to talk to many cyclists before making your choice. i hope this helps.
