I have been a Mastadon fan since day one. I was fan of there stuff long before I ever heard a note based on the name alone. I bought the first album, "Remission" and have stuck with them ever since. Comparing them to Black Sabbath I feel is fair but of course, this is 2010 and they have a sound and approach that is far more theatrical and of the times (obviously.) I am glad you like them and I feel they are a rare find in rock of today. They are currently doing movie soundtracks among other creative works and they are a great example of how sticking to your vision will aid in the music business working for you.
If you like Mastadon, you may also like
The Sword
Douglas Showalter