Well, Pat, I'm biased because I live in Nerja, but the caves here are well worth visiting. It's not a guided visit - you just wander around. The stalactites and stalagmites are spectacular - absolutely huge. One of the caverns is so big that every year (this week in fact) there is a Festival of Dance and Song held in them, with an audience of around 600. Spanish National Ballet and NY City Ballet are amongst those appearing this year.
It'll take you about 90 minutes to see the caves, and there's some good walks in the immeditae area as well.
The Nerja Caves are the 3rd most-visited site in Spain (after the Prado and the Alhambra) so I guess that speaks for their popularity.
If you need to know more, eMail me!
Dave