I've had The Beast and Mephistopheles Stout. I'll be honest, I didn't like either. They were WAY too sweet for me. I figured that that is the way the beer is supposed to be since they're both around 15% alcohol, but they weren't for me. I suggest to taste them warm and with a friend, but drinking the whole bottle may be more than you want to do.
OTOH, some will say they love The Beast. To each his own. Good luck with the tasting!