MM4/5 are an amazing amount of detail, depth, and gameplay for the price. MM6 is pretty huge -- although it is vulnerable to abuse if you know the game too well and want to really burn through it. On the other hand, they're all fun. I still remember playing MM1 twenty years ago, making maps on graph paper. :)
Classic games, tons of fun, substantial replay value. The weakest point of MM6 when it came out was lack of support for hardware accelerated video (because, back then, the pallette tricks they were using for different monsters weren't widely-supported enough, and many cards didn't have the memory for the engine). Given that the game ran silky-smooth on a 200MHz i686, though, there's no worry about that now.
MM6 lets you choose between a real-time system and a sort of semi-turn-based combat system; the latter has some unusual quirks, but it's still playable, and a huge amount of fun to play.
MM7 and MM8 used the same engine, slightly updated, so maybe we'll get lucky and see them some time.