It has some flaws, but I liked it a lot. The fact that the story was about an alien invasion already sounded quite promising, I usually enjoy those, but the plot and characters were even more interesting than I expected. It's not too original but the story kept me hooked throughout the whole game, helped in no small part by the game's remarkable art design: I loved the visuals, it still looks great and the Sixties setting gives it a classic sci-fi movie vibe that I think adds positively to the mix. I especially liked the character's faces, clothes and the alien designs, and some of the environments look amazing even some years after its release.
Gameplay-wise, it's a third person tactical shooter where you can slow time to give orders to a couple of AI companions. There are some quiet moments, you can roam around your base and talk to the people there between missions, but the heart of the game are the encounters with the aliens, which are pretty entertaining. While the tactical options available are relatively simple, they add a nice extra component to the firefights, and the abilities your main character and companions get as they gain experience are quite fun to use. The only problem is you have to watch out for your AI buddies or they'll do something stupid more often than not, but if you make a good use of the tactical view and keep giving them orders things usually work as expected. All in all, I had tons of fun playing.
More info: https://en.wikipedia.org/wiki/The_Bureau:_XCOM_Declassified