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). There are only four civilization (England, France, Spain, and the Dutch) and only one victory condition: independence.
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. Inexpensive, too, at this point.
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. Definitely worth $10 from Amazon, though. OK. Now for my Civ 4 Colonization manifesto: You've read many other reviews stating "don't buy this game" because of two different reasons.
Summary: I am a huge fan of the CIV series and I loved Cid IV. This expansion is the best experience ever. It adds more civs, more techs and some unique little new elements. Such as quests. A little message might pop up saying if you complete a certain number of units or buildings than you win a certain bonus. Also new is weather effects. Your field might get destroyed by tornadoes. The best new element is corporations. You can found a corporation and spread your influence.
Summary: This is an excellent game for those who like a mental workout. Don't expect any easy wins here. It's facinating and engrossing, but also sometimes frustrating. Make sure to give it a good trial.