I have been through this with my recently purchased iMac.
The first one arrived with some scuffs on the side, the replacement had bigger scuffs in the same place. The next replacement had a much louder fan than the other two, so I ended up keeping the first one.
I have still got the option to return that one as well and I might do that but only due to some other software problems with the machine.
I love Apple products but don't trust the QC, and there seem to be quite a few worse problems that people are suffering with here so I am reluctant to send it back and possibly get a worse one.
It is worth noting that Apple have (as usual) been excellent through the whole process, but it is quite a lot of hassle for me to receive and return loads of machines, so I might just cut my losses and keep this one.
If you are in a position where you have someone at home during the day to accept the deliveries, I'd be tempted to keep going until you get a perfect one.