I know I'm late to the party, but just to echo Guilhem, Civ5 is totally worth it. Especially with the expansion packs. It's a big shift in gameplay, but it also works quite well.

About the lack of stacked units, I also missed it a bit from Civ4, but other people didn't like it because it was seen as exploitable, but I liked it because I exploited it to my advantage in single player games :) In any case, after a while I didn't miss it in Civ5 anymore, there's definitely other ways to overpower your AI opponents.

One last thing which the other commenters didn't comment about: graphics. Nostalgic as it might be, Civ5 graphics are more modern than previous Civ games, and although I know a lot of other people say the gameplay is most important, I can't really play past civ games anymore because of the dated graphics. Very shallow of me, but there you go. I still have fond memories of it though; especially the throne room of Civ2. That satisfaction of being able to decorate it... Ahhh nostalgia.