I own a 50 cc Honda Ruckus and love it (shown below). I'm glad to hear a couple other people recommend it.
I bought this 2003 model, four years ago for $900 (it had 1200 miles on it, so it was like new). There is some china clones of the Ruckus, I would avoid them, I've read of horror stories of all the rubber tubing cracking/degrading after 6 months...plus the carburetor issues already mentioned about other China made minibikes.
The ruckus weighs 194 lbs and fit's nicely on a HF aluminum motorcycle carrier that fits a 2" receiver hitch.
The Honda Ruckus has a 4 cycle, liquid cooled engine that is whisper quiet. It has a belt drive with clutches like a snowmobile means no shifting, just twist the throttle and you are moving down the trail. Top speed is around 35 mph with my 275 lb ass on the seat, I'm told that top speed is closer to 45 mph if the driver is closer to 120 lbs, LOL.
ONE big thing about the Honda Ruckus, is it is a very popular design, that hasn't really changed since it came out in 2003, which means there are many aftermarket accessories the fit universally over all the years of production. If I was to ride one regularly "off road", I would get off road tires...there are many different style of tires available in the correct size. I think the standard size would be fine for off road, but I've seen fatter tires/wheels offered for this thing.
Viva Ruckus !