I've got a 2008 Yamaha Grizzly 450. Mostly gets used for carrying about wheat to feed pheasants, towing a trailer on shoot work days, fencing, and suchlike jobs over some pretty steep and muddy fell land. Never gets stuck, never misses a beat, so as long as it's in reasonable nick you won't go wrong with it. Hondas always get rave reviews as well from everyone I've spoken to who has one. Steer clear of diesel ones, they are heavier and apparently less reliable. Kawasaki ones are okay, but not as good, same with Suzuki. Some people have Can Am quads, they do all sorts of engine sizes, you can get two-seater jobs as well, but although they will do the job as a working quad, they're more a lifestyle accessory it seems. I'd stick to Yamaha or Honda.