I played the first one a few years back (Aftermath) and it was average in my opinion. Probably better than the "67" score on Metacritic, but not by leaps and bounds. I was a fan of the original X-Com and played it to completion back in the day, so I was looking forward to Aftermath as something of a "spiritual" successor.
It plays similar to the X-Com series, but the combat is more "live action" than turn-based (or stat-based to be more precise). The battles are actually a lot like the Baldur's gate series, where you can pause during the action to issue new commands to your team members. Things like line-of-sight, training, ranges based on weapon type, etc. play a key part in combat, and you can speed up/slow down the action anytime.
The storyline was interesting, but I felt it stagnated at some point as certain research needed to be completed in order to advance the storyline....but it occassionally took a long time for the research to finish. As a result -you tend to feel like you're "grinding" out a number of encounters just sitting around waiting for things to happen as you're constantly attacked. It's one of those games you quicksave every 30 seconds (at least I did).
I don't mean to be too critical - I did enjoy it, and at the price they're definitely both worth picking up. Never got to the second one (Aftershock) but I think it's probably close in terms of gameplay to the first. Hope that helps!
