Do a lot quad biking here!! however having said that i live in the mountains and can ride all day without touching the road except to cross it if i want to!!! but we often do around 4 or 5 miles on the road to get to various fun locations, my advice?
first do not buy a chinese quad!!!!!!!!!!!!!!!!!!!! crap isn the word!! stick to yamaha/kawasaki/honda etc yes you pay mnore but they are worth it, just for the peace of mind knowing you are going to get home, and the fuel consumtion is damn good!!
anything smaller than a 350 is no good it just doesnt have the punch,here we have a 350 yamaha aircooled it is just up t the job of staying with the 660raptor and the 700cc raptor in the rough, but not on the smooth stuff, and its handy when we get a bit far ahead cos we can stop for a pee/beer/smoke break while we wait for him!! however it is a good learner bike at the same time!! who says they are rubbish on the road? fantastic fun when you know how to drive them, you dont lean or slow down for the bends you power slide round them thanks to no differential, both wheels spinng is great!! the 700 is an animal and climbing hills wil easily tip over backwards unless you know what you are doing!! 660 is a bit more forgiving but can still easily catch you out, although if your reasonable with it you can climb a very steep hill without letting the front wheels touch the ground! scary but fun, have been through floods well over the top of the wheels and somehow it keeps going

servicing is easy but the special oil is expensive, parking brakes have a habit of seizing up if not correctly adjusted, and the slider pin can seize on the rear caliper also, other than that go for it mate they are an absolute hoot if there are two or more of you, but one tip always put a small strap/rope on the q :yuad for towing cos you think nthing can stop them, and then it catches you out!! the 700 got stuck in some deep mud the other day, and i got round him on the 660 and had to reverse back to him to tow him out!! good fun cos i just spun the wheels and covered him from head to foot in thick gloopy mud as punishment for getting stuck!!!