I did the research on power meters in the spring. I have owned Powertap and iBike in the past. My observations:

iBike isn't crap, but it's not at all accurate as you top hills. Everywhere else, I found it to be extremely accurate judged by . . .

Powertap is accurate and relatively inexpensive (compared to the others). It reveals a lower figure than crank-based systems due to drivetrain losses, but you can add the number back in without any problem, if you are so inclined. (I'm not.)

Quarq I have never ridden, but it is reputed to be accurate and easy to service. You can replace the battery in the crank unit yourself. Quarq is more expensive than Powertap, but less expensive than SRM. The problem? They always seem to be out of stock.

SRM is reputed to be the "gold standard". It certainly costs a lot of gold -- very expensive. And, a big downside: When the battery goes dead, you have to ship the unit back to the factory for battery replacement. GAK!

Power data is interesting but, aside from four or five periodic tests during the year, of limited usefuless. I have gone back to daily training by HRM because (contrary to the power meter sales materials) there's not much reason not to. If I rent / borrow a power meter for a total of two or three weeks per year, I will get 90+% of the good I would obtain by owning a power meter year round. [The only thing I really miss is the ability to determine if/when my power drops off for a given heart rate during a series of intervals.]

Using a car racing analogy that I think is pretty apt: A heart rate monitor is the rider's tachometer -- it tells you how fast the motor is running. A power meter is the cyclist's dynomometer -- it tells you how much power you are making at a given rpm. You don't ever want to be without your tach. And, in preparation, it is important to do a few dyno runs -- but not so essential to drag your dyno along with you all the time.