I have been demoing a Medium Epi for about a week now. Ran with a 110mm stem at first and then settled into a 90mm stem for the last few rides.
I am 5'11.5" with a 31.5" inseam. My torso/arms are a bit longish, but not so much that the fit calculator on Competitive Cyclists web page still suggested a TT length of 23.5". Most all other people always said that I should be riding a Large by looking at my body type.
Anyway, I had been riding a Large Ellsworth ID that has a 24.25" TT and it seemed roomy, but it still required a short 90mm stem to feel comfy for me. Anything longer and it was too stretched out. That bike is wonderful, but it doesn't seem to be the right size for me.
With this demo Epi in Medium, I am more agile on the bike. I can bunny hop easier, just like the previous poster mentioned. I can also take drops and small jumps and the bike is just pure balance. The bike is very "flickable" and I have NEVER ridden faster than on this bike.
Medium seems to be the perfect size for me and it could very well be the right size for you. Remember that you can change stem lengths/rise angles to dial in the fit. The right stem makes ALL the difference in the fine tuning for the optimal control of the bike.
Check out
http://www.competitivecyclist.com and measure yourself using their fit calculator. It should give you a really good starting point.
Good Luck,
Jose