Summary: First the bad. Civ4, and now Civ5, is game I can just play over and over again and not get tired of. This is mostly because of the variety of civilizations, victory conditions, strategies, etc. Colonization, by comparison, has a very limited scope (albeit a good one).
Summary: As the title says, this is a good companion to Civilization IV (I still play Civ IV, since Civ V leaves much to be desired). It's a stand-alone game, and you don't have to have Civ IV, to play it. It's an "old" game, but if you've still not acquired it, well, it's worth considering.
Summary: Quick explanation of my rating: 4 out of 5 Stars because it is fun to play and can be addicting. Not 5 stars because of limited number of ways to win; it's not an "every game is different" type of game like Civ 3 & 4. Therefore, I think the long-term playability of this game is limited.