I am a bit biased, as stated above I did have a hand in developing some of the iBike line and still keep in touch with the owner. When I started with the company they sponsored the pro team I was riding for and they had just released Gen 1. I used it for a while, comparing it to my Ergomo I had at the time. When we got our team bikes I had to get rid of the Ergomo (we were also sponsored by Sram, so we couldn't use that bottom bracket).
For a while I used just the Gen 1 and there were definitely issues with chip seal roads and changing slope readings. I called up the owner of iBike and said "Hey, this works pretty well. . .but I can help make it better." By Interbike of that year we released Gen 3 with slope correction, much faster processing (removing filters), and an accelerometer that handles the chip seal roads better.
I know the physics engine was just rewritten (the base code that calculates power) and there are hundreds of measurements every second that are going into determining the power output. One of the cool things I like is that you can start off with the iBike as your power meter and head unit. If you did later on want to get a power tap wheel or a crank based power meter, you still need a head unit to read the info from those and the iBike can do that. A lot of times when people say that a certain power meter is 500-800 dollars that is true, but the head units can be another few hundred bucks on top of that.
I don't officially work for iBike anymore and there are some things that I wish would go into a new product like a better screen and GPS. I know there's a couple things they are working on that are really unique.
Bottom line, I know it works well enough to be a big asset in training. It does require a good set up in order to get good results, and that is where a lot of people had problems and said that it's not accurate.