There are two thing you HAVE TO CHECK OUT before leaving the store or within the first week.
Shoot some shots on ISO400 and use your exposure control to +1 stop to make it clearer, if you have a faulty back there should be stemple like purple artifacts on the picture.
It can be seen on anything above ISO125.
My old back did not have that problem, according to Mamiya NL this is a small problem and it's already been solved, however there are backs out there that still have it, so check that.
Second is shoot tethered for a while.
Make a few shots, fill the buffer, wait a minute or so and go on.
With my NEW back I could not get a stable connection, not on 3 PC's and alot of cables and boosters etc.
According to Mamiya there are some problems but they are mostly due to software related issues and they are solved by using for example :
1. A non powered port
2. A short cable
3. Make a folder for your captures and KEEP that, restart the PC and keep the SAME folder
4. Try to attach ferriet arround the firewire cable
5. Use a mac
I tried all 4 but nothing worked, and I'm not in the possesion of a mac
Here is an example of how the artifacts look at 400ISO
Don't mind the picture it was just shot for fun.