Seems a 50/50 split on get one... don't get one!
For what its worth, I use the dremel corded model. There aren't may applications that I have that don't have power available so its not an issue for me. I have a friend who has the battery pack and he eventually gave up in disgust with it and got the corded model.
Thinking about it though, unless you have a particular application that needs to be cord free, then I reckon the corded one is better value in the longer term. Sooner or later you're going to have to replace the battery pack which isn't cheap... at least its not here in Oz.