Well, I wrote a fairly detailed review of the first one, but the short answer is that they're very easy to get into, have very addictive gameplay, and are very well balanced. They're probably not going to blow you away with story or gameplay variety -- they're not like straight-up RPGs -- but they're a fun way to while away some time. Plus they have strong multiplayer (or at least did, back in the day).
Unlike many of their imitators, the Diablo games tend to retain their challenge and hold your interest longer. Newer games may look better, but they tend to get boring faster. They're a little more 'old-school' and 'arcady' than WOW, which is more social and group-oriented, and have a much darker tone. They don't really play the same way at all. Personally, I prefer them over WOW, which is something I could never really get into. If you get D3 and like it, it's probably worth your while to pick up the previous games.