BestCommentaries.com has gotten a nice redesign—and it added the one feature I most consistently missed. You can now look up a particular commentary set and see how high its individual volumes are ranked. That means you can quickly get an idea how valuable a given set is.
Today, because of Logos Bible Software, buying whole sets can make a lot of sense. Or it can waste a lot of them. The more highly recommended a set is overall, the more likely it is to be worthwhile for you to buy the whole thing rather than individual volumes.
Logos recently came out with an update to the Baker Exegetical Commentary on the New Testament, and BestCommentaries.com helped me decide whether or not to save up for it.