Well - as with anything, you get waht you pay for. There are some really cheap pocket bikes showing up, and my buddy got one for his kid - and him and I trashed it in about 30 minutes
ops:
The forks literally folded in half at the bottom triple clamp after one too many hits on going down the driveway curb.
But we didn't really pay attention to try not to hurt it.
Do yourself a favour and go visit a couple dealers/flea markets and get an idea of the quality and types out there.
They even have them at pepboys now.