I bought my - then 5 year old, now 6 - a Gary Fisher Gamma Ray. We got it at my LBS as they will take it as a trade-in when he out grows it.
I wanted to buy him a Trek (since that's all I own) but he said "I really like the blue one, Dad."
Who can argue with logic like that??
He just learned to ride without training wheels a couple of weeks ago - if you go to
www.youtube.com and type HarlieJ in the search feature, you'll find the video.
A nice bike is the best thing you can buy your kid. Do you ride a junk bike? In all likelihood, probably not. Why not extend it your kid? It builds quality time - I guarantee it. My son had a junk bike and he wanted nothing to do with it. Now he has a nice bike and he takes care of it - and wants to ride with me constantly.
Cough up the change. Buy them a quality bike. You won't regret it.