I have been using power monitoring systems of various types since 2003. I have used the Powertap, the Powertap Pro, SRM and the Ergomo. So, I have a good idea of the nuances of the various powermeters and I have my favorites and my also rans.
The most enjoyable unit I have used has been the Ergomo. I purchased the Ergomo in May of 2007. Prior to that, I had been using the SRM. The Ergomo measures the torque from the bottom bracket, so you will get minimal power loss in the reading. Basically, the power is fairly accurate. Once you get used to calibrating the Ergomo and how the buttons work, the unit is easy to use.
Unlike other units, the Ergomo displays average power on the same screen as your current power when you are in the interval function. This is useful because it gives you sort of a 'carrot' to chase while doing your intervals. The other monitors require you to switch between displays to see the average.
Another interesting feature that the Ergomo has is something called "Intensity Factor" and "Training Stress Score". These two features are a nice way to measure and quantify your training as to how much stress was placed on your body. The two scores are based on your threshold power. The unit measures how much time you spent above and below your threshold power and then gives you a score.
For example, take two rides that are both exactly two hours in duration. The first ride is a recovery ride and the second ride is a hammerfest. In the normal quantification of training, you'd call this two hours in your training diary. At the end of the week, you'd say I got 14 hours in this week. By using the "Intensity Factor" and the "Training Stress Score" you can further quantify the actual stress each ride placed on the body.
Explained: When you determine your threshold power (timetrial power), this is give an "Intensity Factor" score of 1.0. The farther you are below this number, the easier the ride was. The farther away you are above this number, the harder the ride is. This alone allows you to see if you are meeting the goal of your ride. Basically, was your recovery ride really a recovery ride or was your hard ride really that hard? This score will let you know.
The "Training Stress Score" is a bit different. This actually tells you the cumulative effect the two hours had on your body. For example, the two hour recovery ride may give you a score of 50. A two hour hammerfest in the hills may give you a score of 300. The higher the number, the more stress placed on your body. The more stress, the more recovery needed. They give you a generalized guide, but the amount of recovery needed for a particular score is individual and determine by trial and error.
You may think this is way to complicated. It's really not. Instead of using hours or miles to determine your weekly training volume and focusing on increasing hours or miles each week, you can use the "Training Stress Score". For