With the right group it's the best thing that ever happened to humanity, but with most groups it's anywhere from frustrating to pretty good. You need a group of either 4 or 5 people (preferably 5) who can work together and who won't be put off by the real time aspect of the game. It also helps if everyone is OK with losing a lot early on, because you probably will.
Space Alert is quick to play, but the first time you play it you will likely want a good 2-3 hours so you can introduce everyone to the game. It comes with a fantastic training manual that helps you learn a few of the game concepts at a time by playing training missions. Before you even attempt this though you personally should be very familiar with the rules, don't just open the box and try to wing it.
Once you get past that initial training session it is pretty quick, though. We sometimes use it as filler or at the end of the night when there's not quite enough time to play a bigger game. One round of Space Alert takes us 45 minutes, including setup time. It's a hell of a lot of fun with the right people, good luck!