Razor Rip Rider 360 Caster Trike
I got Rip Rider 360 Caster Trike for my son's 6th birthday. My kids love anything with wheels on it, so this was a guaranteed winner. It took my six year old only a few minutes with it to figure out how to really get it spinning while he's riding. The ride is very smooth and he is even able to ride this on our somewhat hilly ride up to the park (unlike his more traditional 'big wheel' type bike which is too hard to ride uphill).
I will point out that if you are buying this for the lower age range, you do need to be fairly tall to ride this bike. My son is 3'11" and is just tall enough to ride this. At first he complained that he was too short, but I think he just got used to stretching a little more than on his other bike. The seat is not adjustable. My almost-4-year-old also likes to try to ride it but is much too short, he just scoots it around pushing his feet on the ground. So if you have a tall 5 or 6 year old, it will probably fit them.
It is still the first bike that gets pulled out of the garage when the kids go out to play, it is just plain fun. I haven't taken it out for a spin myself yet, but it is very tempting.
(This is just some reviews from real customers.)