I have close to the opposite experience, I find the strap works really well over the shoulder.
I ordered a fake A & A bag which came already, I can tell you, that strap is VERY thin. To me, it hurts like hell without padding. I had to put a domke postman strap pad to stop it from hurting. I guess your experience might be different. It doesn't slide off though, but I found neither of the bags did.
Also the fake A & A is very rectangular, almost like a video camera bag. Not really my preferred shape, but I have been told by numerous females it looks "very formal" when compared to the crumpler. Something you would expect someone at work in an office or a wedding to wear. Good or bad depending on the look you're going for I guess. Still, depsite the strap, I like it. I'm pretty sure the top flap is real leather but I'm not 100%, I mean, it is a knock off from China so who knows. It smells real though. Also, the bag is about the same width & length as the 5 million but about half the height.
Anyways, hope you have better luck with the fake A & A when it comes!