My brother and I bought a Technet subscription a couple years ago and we're still using the Vista Ultimate x64 license from it. I've probably installed the OS 3-4 times personally (through 2 or 3 systems) and he's used it 2-3 times for his two PCs. Its never given us an issue, even after our one year Technet subscription expired.
I also got a copy of XP Pro, Vista Business, Office 2007 (which is the worst office program I've ever used) and a crap load of other software and dozens of license keys. Not bad for $350, which my brother and I split.
I highly recommend a Technet subscription if you need a bunch of software, but the upgrade preorder prices for Windows 7 right now are hard to beat if you just want to go from Vista to 7.