My friends and I have spent many MANY nights and weekends playing this thing for hours on end. The craziest thing about this game in particular is its range of appeal. Half of my regular Munchkin players don't play DnD. They've been approached by me, and they turned it down. It's probably because I didn't shower and was basically begging them, but I'm going to just chalk it up to them drawing the line of nerdiness at Munchkin.
And thats perfectly fine. Its a great way to merge two groups of friends: DnD regulars, and their less nerdy counterparts. Its simple enough that anyone can learn it in about 20 minutes or through a quick demo game. Its complicated and subtle enough that you can strategerize and barter and bicker about rules all night long. Its goofy enough that its a great ice breaker, and encourages side conversations.
There are TONS of expansions for this game too. Each one has its own theme, like James Bond, Westerns, Space, of Cthulu. However, all these cards can be freely mixed and matched across the different versions. So if you love Steve Jackson enough and buy everything he's done, you basically have two jenga towers of cards from which to draw. And a whole table of people to hate you if you knock it over.
Here are some Youtube videos I found that really explain the basic rules and concepts better than I could. So I leave you in the capable hands of Scuba Ducky and Camo Ducky. These videos are NOT me, by the way. I'm not lucky enough to own two ducks like that.
Throw a couple drinks in folks and you are REALLY looking for a good time with this thing. Especially with couples. You can barter and exchange services and real world goods for cards in the game if you really want, and well, couples are better at that than most if you catch my drift.
Definitely give Munchkin a try. Its fun, quick to learn, and for some inexplicable reason, everyone wants to play it. Ive seen some other reviewers call it the "Gateway Game." And that is 100% correct. To throw in extra nerdiness, I use d10's to keep track of everyone's level. Its not like they are useful to me for anything else.