I have used JVision, Rational, and Together Control Center. While Rational and Together have lots-o-features, I like JVision because it's simple yet I can effectively create the diagrams I want.
One thing that I find irritating in Rational, is that it takes a couple of minutes to start up (on a Pentium 866 with 512 RAM, without any other applications open!). Also, it seems to take quite a while to save models, etc. Installation sucked also. A big 'ol red flag for me was the Installation Manual. If you have to write a frickin manual just to install the tool...
The only "rational" reason the company I work for chose Rational, is because it was cheaper than Together. I think you only get what you pay for. (OK, I'm done ranting.)
[This message has been edited by Glen Tanner (edited July 26, 2001).]