First there was UFO: Aftermath, which was a limited recreation of the old X-Com games. I never finished it because frustrating and repetitive gameplay had me tearing my hair out by the endgame.
Then came UFO: Aftershock, which was also a limited recreation of the old X-Com games but a bit closer this time. I never finished it because frustrating and repetitive gameplay had me tearing my hair out by the endgame. I did get ALMOST to the end though and it wasn't nearly as annoying as the first game.
Finally, so far, came UFO: Afterlight, an almost complete recreation of the old X-Com games with extra bits. The fairly pedestrian graphics of the original two game have been replaced with a more cartoony feel and there's more to do on the strategic map. I finished it and came very close to enjoying myself for most of the game. Like the other two games it's highly repetitive but it's also tactically evolved and with a more interesting storyline.
If you're a completeness freak then this is the collection for you but if you haven't sampled the UFO: franchise then I'd skip the first two and look for the cheapest copy of Afterlight that you can find.
All the games had a pretty active modding community so be sure to check out the fansites.