Rich1988 covered pretty much all the points I think, worth heading over the
http://www.400greybike.info/newforum/index.php which is a general 400 forum but a very active V4 section for the honda's.
I do love them, not ridden mine on the road for a while now as it's a racebike now, but looking to make one up from spares for the road again (I recently bought a second race bike and more spares than I know where to keep
)
Most parts are still available brand new, if not pattern or replacement parts are available (eg lower rad, original honda stuff is fairly rare, but although it's a bit costly at ~£300 Rick Oliver makes a brand new lower rad, which is actually better than stock, got one of his upper rads and it's very good quality), so getting parts is easy,
Maintenence however is a bit 'meh', some things are fine, others are a pita due to the V4 being crammed in, as an example I'm about to replace the front head on mine due to a knackered exhaust stud, can't do that with the engine in situ, but probably could do the rear head. but that depends on whether you'd actually be doing your own maintenence of course