The Baby Bjorn (http://www.amazon.com/Baby-Bjorn-055121US-BABYBJÖRN-Potty...) is my favorite, hands down. I bought a Safety 1st one initially, and I'm pretty sure it was one of the problems because that stupid shield kept popping off every time he sat down, dropping into the potty. Sometimes it would pop off while he was getting back up afterwards too, so ... eww!
Also, the Bjorn one is the only one I could find that is shaped like all the ones on the videos we watched, which also helped. When we were doing our big potty "party" day I did some online checking and decided it was worth it to switch to the Bjorn. I have not regretted it at all. He could just sit down on the thing and go without having any hassle with the lid or shield or anything. And it IS easier to clean than the ones that pull out like a drawer.
Oh, I also picked up the Fisher Price (http://www.amazon.com/Fisher-Price-L1496-Fisher-Price-On-...) travel potty. I like that one too. Very convenient for on the road, and I've had no troubles getting my son to use it when we've needed to. I'll probably get more use out of that one, technically, as he's already moved over to using our adult potty when we're at home.