I stand to be corrected, but AFAIK the OS and Office keys don't expire (outside of the 10 activations /key limitation), but the higher end software (like the business server apps) keys are only good for as long as the subscription is valid. If you're mostly looking for OS and Office, then you won't find a better deal. Just keep in mind that it's for your own personal use... I'd imagine MS would take a dim view of things if these keys start showing up as updating from 100's of different IPs (as in you're building systems and putting them out the door with these keys).
Are you sure about the 10 activations limit per key? I think that's kind of scary as I upgrade various parts in my computer throughout the year and that may trigger an activation request. What are the odds MS will deny an activation? I'm really new to this and I guess I'll have to more or less take your words on this.