Previous versions of Windows would allow you to use the "upgrade" disk to do a clean install; it would ask you to insert on old OS disk to prove you qualified to use the upgrade disk. 7 doesn't do this. If you try using the upgrade disk on a blank hard drive, it gives no warnings and doesn't ask for any proof that you own an older version, it just goes ahead with the install. But when it asks for the key, it will reject the upgrade key without explanation. I eventually discovered that it was looking for a full version (i.e. full price) key.
This means if you're trying to install Windows 7 to a blank hard drive (e.g. replacing a dead hard drive), you have to install your old OS or run the computer's recovery CD first. Or you can do like I did, and install 7 a second time; as long as the installer sees an installation of XP, Vista or 7 (even if it's unlicensed) already on the drive, it will let you use the upgrade key. Choose a custom install and the installer will rename the old OS folder to windows.old before creating a new clean isntall, you can delete the old OS folder once the install has finished.
Other than the little hiccup with the keys, 7 installed very easily. But then I started having problems installing software. The installers would freeze up, the computer would start acting oddly, then when I tried to shut down, the machine would freeze up. I couldn't believe Microsoft could have released an OS this buggy, especially not after everyone had been raving about how great the beta was, so I figured I had a hardware problem somewhere. I also noticed that the Windows Experience Index, which tests your hardware's performance, was giving me excellent numbers on my CPU and video card (7.0-7.1) but a horrible score for my hard drive (2.8). On a hunch, I broke my RAID mirror and the hard drive performance jumped to 5.9. Some of the gremlins also disappeared as well. I don't like going without RAID, so I installed a PCI-e RAID controller card and disabled the motherboard RAID. That did the trick; all the problems went away. Unfortunately disabling the onboard RAID also disables the eSATA port, which I was using for an external hard drive, so I've hard to order more parts to fix that.
With the hardware issues cleared up, 7 seems to be working quite well. UAC, one of the biggest complaints about Vista, is no longer annoying. The box came with both 32 and 64 bit versions; the 32 bit version has better driver compatability but is limited to about 3.5 GB of usable RAM. The 64 bit version can use up to 192 GB of RAM (!) but some programs and drivers won't work. So far I've not encountered too many problems, other than the fact there's no version of Flash for 64 bit IE. (Buh? 64 bit Windows have been around since XP, in 2001, why has it taken Adobe 9 years to come up with a compatible version?)
Startup does seem a bit faster, but that could just be because many of the services are set to delayed-start. There are lots of little tweaks to the GUI, some good, some not so great. The biggest improvement in my opinion is with Windows Media Center. I have a TV tuner card and I use WMC to record and watch programs. The Vista version of WMC couldn't deal with digital subchannels (12.2, 12.3 etc.) very well, you had to add the channels manually and you'd never get Guide info. 7 has full support for digital TV and provides complete Guide information for all the channels, so now I can record things on the local broadcast movie channel.
You never really appreciate just how much software you have until you have to reinstall all of it.