We've had this kart for 2 years now. It is still running great. I got it for my then 9year old boy. Awesome Christmas surprise! He is 11 now and still enjoys it and uses it although at this point he wishes it were faster.
No worries, however, little brother at 6 years old loves it and it is plenty fast for him. He is young and I have to remind him not to show off or get crazy with his driving. So far so good. This is where parent discretion is advised. We have lots of boys in the neighborhood, a neighbor (8yrs old) who had ridden the cart a few times before suddenly crashed straight into the curb at top speed. I was there watching and he wasnt "driving crazy" or anything.. he bent the front wheel bar (husband fixed) and the boy got some scratches. He hasn't wanted to ride it again-I'm ok with that.
While not super fast, it is fast enough for trouble, low profile enough to drive under parked trucks and smash heads ( hasn't happened here but I saw the potential). Also the low profile and muted colored flag makes it hard to be seen by cars. Driver error can happen no matter what the age of the kid. If your kid is crash prone or a distracted type, or a stunt show-off, injuries are going to happen. If your kid is a high powered driver of all things motorized with some sense of self preservation- this kart is perfect for ages 5-9. After that your tween will want something faster.
NOTE: The kart does not drive at all on grass and is uncomfortable over large cracks or uneven ledges in sidewalks. It does not climb inclined slopes either. Storage below freezing= dead battery.
But we still love and recommend it. Fun on smooth paths and cul de sac play. Easy to drive. I'm 5'3" 125lbs and I can drive it- although it's slower with me on it.

Decent little kart. Build quality isn't too bad, though I did have a failure fairly early on. I bought this for my son for Christmas and after about 12 days or so, 7-8 recharge cycles and 10-12 good long rides, it quit. I went online and found a somewhat local authorized dealer that was able to trouble shoot and repair a faulty speed control on the spot. With proof of purchase (Amazon email) I walked out of the shop same day and didn't pay a dime under warranty - thanks to EZ Green Electric Bicycles!!

Please note that this kart is called a DRIFTER kart. Razor has intentionally put rubber tires on the front and HARD PLASTIC tires on the back, which allow it to over steer like WHOA! The plastic tires wear quickly (wore out the first set inside of 2 weeks, $20 for a pair) and are VERY LOUD. You can hear it being ridden from down the block. As they wear, they become out of round making for a rough ride. My son complained about his entire body tingling after he gets off from the vibration, but he loves it anyway.

Make sure that your child wears the supplied seat belt, as the side to side forces encountered when you yank the wheel hard to one side at top speed sends the kart into a high G loop that can toss the rider out of the kart, but that's part of what makes it fun. One unintentional side effect of this kart is that my son has absolutely mastered drifting (the reason the rear tires wear so fast) and I think it will actually help his real driving in the future if he ever encounters rain, snow or ice.

Minus 1 star for the premature speed control failure.

The first was broken. The first we received did not work because the chain drive was not connected to the sprocket underneath the cart. It was a hassle to have to repackage the cart, load it into the car ( it was heavy )and drive an hour to a UPS store to return it. We have a new cart now and it seems to be working fine.

My son loves it and once ur strapped in it is actually really safe. Won't flip and not crazy fast

got it last year x-mas when my son was only 5. He loved it from a moment He sat in it. It is a most fun for kids. It is worked fine, no problems
and the fast wear on a rear tires are not so fast. He drifts all the time, no spinning and then stopping, he does drifts.

However just about a year later (and approximately 30 days of use) the batteries are dead. They won't take the charge anymore. We always followed the min. 12h charge time but I admit I might have let them on for longer. Who can remember these things ?
There should be a shut-off switch once it is charged. Anyway I got two batteries from amazon. Installed them. They fit right, but two weeks later they are dead. No charge. Emailed seller he sends one new one as he says tow can not be dead, must be one of them. I tried switching them, no results. The new batteries last half the time and 80% of the speed.
Then I ordered a pair of new Razor batteries for twice the price of the generic. They do work great. Super.
6 months later and about 45 days of use they are dead again.

So I had enough of the short run times - 40-45 minutes of maximum run time after a 12h charge, so I'm converting it to gas power. Unfortunately gas power is still the future until battery technology does not improve , a LOT.

But the kart is great. After so much use there is still some life left of the drifter tires (plastic) I just wish that the non-drift and drift tires/wheels would be interchangeable. They are NOT ! Some days He just wants to go fast and no drifting.

I experimented with melting a little thread into the spare (very worn) wheels. They worked very little as the trouble is not the lack of thread but rather the material - they are plastic tires.

My son loves the drifting but at times it would be good to just go fast. So I will try and get a solution to that after it is a gas powered machine.

Update - 12/05/2015

Gas conversion is done.. It is awesome. Had some issues with the chain, brakes, fitting of a gas tank, etc. but it is done. Well the kart is about 40% faster and about 250% louder. New trouble though, the rear drift wheels are gone in a short hour. Of course with the electric engine nobody drives these things hard for a long time, battery life will prevent it. The gas engine just keeps on going and tires are getting super thin.
My son now almost 7 drifts like a pro. Keeps the drift for a long time and counter corrects when needed etc. Perfect drifter thanks to the kart but the rear wheels/tires needs to be replaced pretty soon.
So the good ol' gas engine conversion will keep the kart alive despite the weak and expensive batteries.
Side note - neighbor purchased same kart, batteries lasted exactly one year for them. They used it maybe 15-20 times in that year. We used it more and still lasted 12 months only.