I've had an Etrex 30 since they launched, so 4 or 5 years now. I use it on the bike, for walking and for ski touring, and I'm really pleased with it. It does everything I need and more, and the sensitivity and accuracy is pretty impressive, so much better than the Geko it replaced.
Jim's right about the differences between the 30 and the 20, and tbh if I needed to replace my 30 I'm not sure if I'd settle for a 20. It would be a hard choice. The built-in compass is occasionally useful but only through lazyness to avoid pulling out my proper compass. I do find the altitude reading on the 30 are remarkably good (this always used to be the weak point of GPS receivers) but I don't know to what extent that's just the improvement in the receivers or how much the combination with a barometric altimeter improves it.
The only downside for me is that my first unit had to be replaced when it stopped recognising the presence of the mapping SD card. But since Garmin replaced it under warranty I can't really complain.