Buy the base game, but not the DLC (The Unicorn and the Falcon or something). I loved MMX, but be prepared that it's more like World of Xeen than it is like the second trilogy. I found the difficulty level nearly perfect. I made the mistake of playing through the DLC directly afterward and it left a bad taste in my mouth, sort of tainting my memory of the main game.I didn't experience any of the game-breaking bugs Ossie mentioned. In fact, I encountered no obvious bugs at all.
I strongly recommend these two mods:
http://www.nexusmods.com/mightandmagicx/mods/1/?
http://www.nexusmods.com/mightandmagicx/mods/3/?
The class skill rebalancing really corrects the shortcomings of the class designs.
