I built my new machine about a month ago and for the OS decided to go with the 64-bit Windows 7 RC. I'd heard great things about it and had tried it out myself on my old system under a virtual machine and figured that it was stable enough to try out in my new main system.
I definitely made the right decision.
Windows 7 is everything that Vista should have been but wasn't. Vista's downfalls have been detailed in plenty of other places so I feel no need to rehash them here, other than to say 7 is basically Vista under the hood but has the ease of use, flexibility and performance of that old workhorse, Windows XP.
Microsoft definitely figured out that Vista was piss. It had horrendous driver support at release, performed worse than XP on a like-for-like system and crashed like Mel Gibson headed for the local anti-Semite rally (OK I lied about the rehashing part). By comparison, 7's driver support is amazing months ahead of its release (it picked up all of my hardware at install, with the odd exception of my Creative X-Fi Xtremegamer PCI sound card - but Creative had beta Windows 7 drivers on their site which worked just fine), it never crashes and my performance is through the roof. The only bad thing is that it does not make me strawberry milk or look like Liv Tyler.
I've got all of my games running under 7 with virtually no issues. WoW, TF2, L4D, Mass Effect, Prototype, World of Goo, Aion, Battleforge, Champions Online, Guild Wars and more are all running flawlessly. Well, one flaw at least. Sometimes when I join a server in TF2 it will hang, for seemingly no reason at all, and give me the deliciously-generic message of "hl2.exe has stopped responding". This is the only game this happens on, and a Google search reveals that this is fairly common so I'm willing to chalk it up to a TF2 oddity. Everything else runs perfectly though, with the exception of Prototype which has slowdowns but since these have been reported on non-7 machines then I do not believe the OS is to blame.
Perhaps one of the best things is that the RC ("Release Candidate" - think "advanced beta") is completely free to use through next summer. Around next May/June it will start whining to you about how it is about to expire and will then limit your sessions to two hours (forcing you to reboot) before it finally dies. Between now and then you get the full version of (what will be) the best release of Windows to date, for nothing.
To start off, head over to http://www.microsoft.com/windows/windows-7/download.aspx where you can download the full OS (in 32-bit or 64-bit flavours, I recommend the 64 unless you're certain your system has components that cannot handle it) and pick up a product key to activate it. Hurry up though, because Microsoft has announced that the download will be disabled on 15th August. It costs you nothing so what do you have to lose?
The release candidate of Windows 7 has been so well received (and is largely bug-free, almost a rarity in today's software marketplace) that Microsoft recently announced that it will go retail on Thursday, 22nd October. By using the RC now you get the best version of Windows 7 (Ultimate - all the trimmings) for free for the best part of a year.
That's a hard deal to beat.