Parts are hard to find (over here in NZ anyway!), but there are quite a few of them about. If its in good running condition then they seem to go pretty well especially with younger riders, as they are have very mild power, and don't tend to get away as much as the modern bikes.
The main thing I didn't like about the one I rode is that they have completely different steering geometry to the modern bikes, and I had lots of trouble doing slow full lock turns, but this may have just been the bike I rode (at my level we do a lot of off camber tight turns).
It really depends on the condition of the bike as to whether its worth getting. A friend of mine bought one really cheap a couple of years ago (it came in two cardboard boxes), but it took forever for him to get it to the stage where it was working effectively, cost him lots of money, and he didn't really enjoy riding it anyway, so he bought a '97 Montesa instead. Dave Trustrum also had one, which he got fed up with, and then went and bought a 2000 GasGas 200.
I would also talk to the Beta importers about parts availability and also ask any questions on
http://www.trails.co.uk in the bulletin boards.