I used to use a reversed 50mm pentax lens on my sigma 105 before getting an MPE-65. Since then I've learnt a bit more about high mag shooting so decided to do a quick and dirty comparison of the 2 at 3:1. Using my 350D/sigma 105 with a reversed 50mm pentax lens at F7.1 and a 40D/MPE-65. I'd normally shoot around F7.1 at 3:1 - I used to shoot F11 when using the old combo before.
The target was the queens eye on a £20 note. The whole thing done handheld resting the cameras on a table so I can't guarantee absolute reproduction of shooting angles.
My conclusion was that the sigma 105/50mm combo does not have as flat a field of focus (important for 2-D shooting but not so much for 3-D subjects) as the MPE-65 (shooting angle might be different as well), but where both images are in sharp focus the resolution seems very comparable. One area I do think is different is the handling of OOF areas where the bokeh of the MPE-65 seems better. The focus distance of the 105/reversed 50mm was also a lot closer than the MPE-65.
Does show though that as others have demonstrated you can get excellent images with a reversed (old) 50mm lens and it's raining outside at the moment .
#1 350D/sigma 105/reversed 50mm pentax lens F7.1
#2 40D/MPE-65 F7.1
#3 100% crop of #1
#4 100% crop of #2