I have tried a few BA bags.
The good: I love the system. I am a side, stomach, back sleeper and roll all night. Being able to do so in the sack without falling off the pad is WONDERFUL. Their customer service, at least from a product question stand point, is awesome. Really nice, responsive and personable. Bag quality seems good, on par with most imported bags particularly in the higher end bags. Not WM, but nowhere near the cost.
The bad: Although I'm not the best judge of temp because I am a freakishly cold sleeper, I think they are somewhat optimistic in their temperature ratings. In particular, sleeping directly on an insulated air core was a very cold experience for me in the 20s with a 0 degree bag. Those large sidewalls seem to let the pad chill down quite a bit. If you go with their mummy bag, you MUST use their pad which are either heavy for the warmer ones or cold for me. I haven't tried a POE so that might work but REI's Lite-Core doesn't nor does the Prolite or at least not without odd stuffing or folding. If the pad doesn't fit correctly, you can leave gaps of just nylon rather than pad or insulation allowing heat to seep out.
I think the functional girth is not as large as the specs indicate. Stuffing the pad in the sleeve stretches the bag out and makes for a tighter fit than you'd think given the dimensions.
Like I said, I sleep WAY cold so my temperature experiences are probably extreme compared to the "average" user. I chose to get rid of mine and invest in a -10 WM bag for all around use. No joke. Our scoutmaster has a -20 and complained of cold at ten degrees but I don't know what he was using for a pad. My friend recently bought a Lost Ranger semi-rectangular pad rated for 15 degrees and was fine in the temperatures I was cold in while also using an IAC pad so ymmv.
If I could make it work, I'd still be using it due to the fixed pad thing but I need to make sure I keep warm and don't think I can get a bag from them matching the weight of the WM that will provide the same warmth.