We have one of those seats, too, but the problem we have is it makes it impossible to pee into the toilet bowl while sitting [even with the pee guard]. So I haven't been putting it on the toilet lately and my son doesn't mind...he just spreads his legs wide enough to sit on the big potty and holds himself up with his hands. Since he is sitting so far back [most of his butt fits on the back of the seat], there's no problem with getting his pee in the bowl either. He has fallen into the toilet only once and it wasn't a big deal for him.
One thing that was a big deal, however, was one time when he pooped and it splashed water back on him. It caught him off guard and scared him a little I guess, and he would not go poop on the potty for weeks after that happened.
Another issue I had was, when we would go to the bathroom and he went potty, we'd wash our hands, but he would always want to keep playing with the water in the sink or start whining to take a bath. So every time there was a big struggle to get him out of the bathroom. And he would also ask to go potty over and over again [I always took him because I never wanted to refuse to let him go potty!!!] even when he had no intention of going potty or even sitting on the toilet... just to get in there to play. THAT was the reason I decided to get a small potty chair. I still keep one in the living room so that when I suspect he is just manipulating me to get into the bathroom, I can pull it out and say "Okay, here you go, you can go potty right here" and if he truly needs to go potty, he is happy with going on that seat too. [I got him this Fisher Price "Froggy Friend Potty" chair which is similar to the Bjorn potty chairs and it is so great and only $13: http://www.walmart.com/ip/Fisher-Price-Precious-Planet-Fr... ]