A nasty is that MS has stopped supplying the keys with a laminated top protection, giving just a paper label with the 3M VHB adhesive and the "void" precut tears in it. Thus after a month or two the OEM serial key will wear off from the label, so if the HDD goes south you will look and find the key is unreadable.
The OEM keys are easy to get, just buy a MS mouse and keyboard, and this qualifies you. Of course they also suffer from the BLACK SCREEN OF FAIL every so often, especially on a computer that has not been turned on for a few months. Bit of a pain if you actually do have a volume key for Windows and Office, and it is paid up to date.