It's pretty straight forward to figure out whether using a RAM disk is worth it. A ram disk offers the possibility to have the FASTEST DISK your Mac could ever have. Therefore it's the best solution for applications which require frequent disk accesses.
Moreover Ram Disks are very useful for Powerbooks and iBooks because using a Ram Disk instead of the hard disk means using MUCH less power, therefore dramatically increase battery life. For instance a Powerbook can play 2 hours DivX by placing the movie on a ram disk because there will be only few disk accesses.
Ram Disk Creator is a shareware application. However you can use it with all features enabled and fully evaluate its capabilities.