Walen: Are the sequels any more memorable?
JMich: UFO: Aftermath is the weakest of the three. Aftershock and Afterlight add quite a few elements to the game, including a bit more in-depth base building (other than research/engineering/military that the first offered), more enemy types, including humans and aliens galore, weapon customization, separation of burst and aiming levels and quite a few more mods (though not as many as Jagged Alliance 2). If you enjoyed the first installment, UFO:AS and UFO:AL are worth it, since they improve on it. If you found UFO:AM as "meh", then you may or may not enjoy the other two.
blotunga: they were an X-Com clone
JMich: They are not X-Com clones. The main story idea is the same (alien invasion, defend, adapt, count-attack), but one game is TB the other RTwP. Calling the UFO:A? series an X-Com clone is akin to calling "The Walking Dead" a "Dawn of the Dead" clone.
What confuses me is that there's ALSO this thing called 'UFO: Extraterrestrials' that's unrelated to those other UFO games but still looks like an exact XCom remake: