We went through this with two of our three sons.
My oldest always had crappy bikes that fell apart. Something was always busted. As a result, he didn't ride much.
Then I broke down and spend $250 or so on a nicer Haro bike for him. Nothing fancy - fixed gear, no fenders, no nothing. He rides that every single day. To and from school, through the neighborhood, etc. (He's 9 now)
Having learned that lesson, we bought my 5yr old a nicer bike to start with. He got a small Torker from a LBS that he LOVES LOVES LOVES. Rides it every day. It's probably his favorite thing that he has. The training wheels are worn to a nub. The rear tire has bald spots from all the skidmarks he puts down.
Neither bike has needed fixing, they just work.