First off, the basic Fluxx game mechanic is a 4-star game. It's fun, trivially easy to teach and not too frustrating for the players. But in its "beer and pretzel" silliness are mechanics that let it sometimes get bogged down or too weird.
Pros:
It's very easy to play, even for beginners--starting with a single "draw one, play one" rule.
It's silly fun with half the cards changing the rules or the goals of play.
It scales well from 2 to many players
It is somewhat replayable
Normal games are about 15 minutes.
Cons:
It's hard to use very much strategy
It's HIGHLY variable in length. One game can be a 3-card laydown win. Others are an hour or more.
It's good for about 3-4 games before you're moving on to other things.
The "Pirate Fluxx" deck is a stand-alone game which has the same basic pros and cons, while being marginally more fun than the basic game. It contains a full deck of cards specific to Pirates, and introduces new concepts along with the new theme. A few key cards of note:
The Captain's Hat, adds bragging rights to the game and a few play variations.
The "talk like a pirate" card is fun, though gets silly after a while.
Two new creepers are interesting, easily gotten rid of, and add a bit of variation to play
The "Plunder" card introduces significant card swapping among players.
If you've played Fluxx before, or want a safe first time purchase, it's worthwhile. It won't make the sole basis of a 4 hour game night, but is a good filler.
Of the theme decks I've played so far (Original, Monty Python, Zombie, Pirates) Pirates is my favorite.