There are plenty of folks using the OEM version.
In years past, all of us that built our own systems used OEM copies. MS about a year ago started trying to change this and has even gone so far as to delete info around the net where they said this was OK.
I have seen as recently as a couple of months ago, MS certified folks on Microsoft's own forum say that it is ok. So, there is a lot of conflicting information happening even now from MS related to this issue.
The one thing to note with an OEM, is that once installed and activated on a computer, that copy is basically tied to that computer. You must also provide your own tech support.
If you might need to move it from machine to machine (of course only one at a time) and/or want support from MS, then the full retail version would probably suit you better.