Interestingly, I'm in exactly the same boat. Stock > Dinan stage 1 > Benvo/BPMSport.
Vin's opinions above are very similar to my thoughts, but I will reserve any extensive commentary until I have had a chance to really spend some more time at full throttle above 5 or 6k. The weather in the PNW has been really wet and somewhat cold - it really hasn't stopped raining for the past few weeks (I think Mike doesn't believe me!).
Why did I make the switch? I'm awaiting the arrival and installation of my Turner Test Pipes, and thus I needed a different tune to take care of the CEL. As with Vin, I was really pleased with my coding with Mike, and thus I became a repeat customer.
The BPMSport tune thus far has been great with a smoother and more linear throttle response, lots of great mid-range torque b/w 3-5k, with a much-improved cold-start sequence and driveability. My limited time above 6k thus far has been truly eye-opening, with a sensation of lifting off the road due to the new-found thrust. I can't wait to drive it more, especially once the weather improves, the summer setup goes back on, and the test pipes are installed!
2011.5 E90 Dinan M3, SS II, M-DCT, EDC, M Drive. Winter setup 220M w/ 255/275 Blizzaks
Performance Mods: Dinan St.1 suspension, exhaust, intake; TTPs; BPM St.II, DCT, servo tunes
Cosmetic Mods: OEM CF splitters, mirrors, center console, spoiler; Arkym CF diffuser; LUX; tint