I don't think they are inherently good or inherently bad. I believe they have pros and cons.
From my perspective, the biggest pro is having a clear picture of where things are. Yes, it's a game of the imagination. Yes, the GM is describing things. However, there are times when my description of something seems clear to me, but may not be clear to someone else. Also, as DannyAlcatraz said, "they minimize disagreements about who was where and when." On a more personal level, I find painting to be something which helps me relax, so it's nice to have a practical use for the things I paint (minis.)
The biggest con I've noticed is that a lot of people I game with tie themselves too heavily to the battlemat, and sometimes this leads to virtually every encounter being restricted to areas which are the size of the table/game space being used. I've had a few experiences in which I tried to abstract things by using minis and the battle map as just a rough illustration combined with a verbal description, but it was lost on some of the players involved because they were so accustomed to minis being an exact representation of what's going on. I'm not so sure that's something which is a product of using minis... I mean, it is, but I believe it's more an issue of a game situation which was different than what those particular players had experienced before.
Overall, I think minis enhance the experience. Even if you don't have minis, just using some checkers, chess pieces, or whatever you have and a quick sketch can help to illustrate a situation. Where they become a problem (I believe,) is when you become so tied to them that you lose your ability to expand your game beyond the space available on your table.