Novro,

Just a few thoughts on the topic:

There seems to be quite a variety of quads appearing on the market nowadays, here in the UK at least - I get the impression that they are growing in popularity.
Everything is available from "toys" through farmers' utility vehicles (you don't see many farmers riding bikes in the UK nowadays) to the more off-road, distance-capable vehicles.

Lots of manufacturers out there, including the major Jap names for two wheels.

The 3 wheel variety (1 wheel at the front) are now illegal in the UK, which is just as well because they certainly behave as described by henry - I agree with the premise that "bailing off" a bike when crashing is easier than with any other form of mechanical transport.
The 4 wheel design can be easy enough to turn over on side slopes, just as tractors can do, and this has been a common way of injuring/killing (d)riders. (you can have a roll bar fitted of course).

Accidents with them get quite a bit of publicity as well: novelty value perhaps, combined with the ubiquitous celebs who have been known to kill or seriously injure themselves while riding a quad, here in the UK anyway.

There is another thread somewhere in here about a proposal to take a ride on quads in Africa - there were quite a few posts in there IIRC.
There was also some comment about whether quads should appear in here at all (but not a patch compared with the subject of sidecars!) - therefore, you may find other posts on this topic in the 4 wheel section of HU.

I have read a webpage (sorry, I don't have the url) about some Canadians who are riding quads from there north to south across the USA, all off-road: IIRC, they are using Polaris quads and they have some sponsorship - nice if you can get it!
My guess is that there is still some novelty value to possible sponsorship of quad riding, whereas it seems to me that sponsoring bike riding is pretty "old hat" now.

There is another website about a guy who is driving around south America with a beach buggy, based on a VW beetle (made in Brazil no doubt).

Cheers,

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Dave