As someone who games with my partner, I always like to know how a game plays for two players. Fluxx was one of the first games we played together, since it was one of her favorites and I wasn't hauling my game bag to her house to impres her with my nerdiness...yet.
We have a balance that seems pretty typical with gaming couples - I like longer, more strategy-based games, and she likes shorter, tactics-based games that are boring...I mean, different.
Fluxx is a fine a middle ground. Here's what I found to like:
+The 70's style art and descriptions are fun.
+There's a little strategy you can inject if you want to give yourself something to do while flipping over cards.
+It can be a good filler game while waiting around or out-and-about.
+Pretty fun modifying the rules and making up crazy card-drawing or discarding to foil your partner...even if you don't win.
+Easy to introduce to friends. Wouldn't quite call it a gateway since it doesn't open a gate toward strategy games...
Here's what I don't like:
-Very little strategy. Mostly luck. What you get, you get.
-Not too much to do after you know the cards. Game goes from blind luck, to a little knowledge of what card combos can win, to boring.
Specifics to two-players:
+Not radically different from 2 to 3 or 4...you don't feel like you're missing out too much.
-Can feel a little aggressive since you can are directly attacking another player with your rule choices.
-Doesn't have as much randomness as with more players.
The pros may outweigh the cons in number, but not in significance. After we got deeply, scarily addicted to Race for the Galaxy, and really after I wouldn't play Fluxx without groaning and rolling my eyes, this game started to gather dust. However, if you're wanting a light filler that's quick for two and easy on the brain, it can definitely fill in a gap.