I've been using Windows 7 RTM for a few days, so I wanted to give my thoughts about it running on my home development laptop (Core 2 Duo 2,2GHz, 2 GB RAM, NVidia 8600M GT).
I haven't tried any betas or RCs previously (mostly because I don't want to make free betatesting if I can't even upgrade from RC to RTM), and I have Vistas (32 and 64 bits), XPs (32 bits) and a Windows 2003 Server machines.
I won't enter into stupid "how to install" howtos or dumb tutorials, so I'll just get to the points I think matter.
Install process is almost the same as Vista, nothing really new here. It installs more or less fast (around 20 minutes on my laptop). I didn't check how big was the Windows folder just after installing, but right now it weights 10,5 GB.
I Only had to install NVidia display drivers for Windows 7, all the rest detected perfectly, and for things like my USB Scanner or my Windows Mobile PDA, Vista drivers work perfectly.
First thing I do when clean installing an OS is remove unwanted features, and here I can only say: Almost every option is finally somewhere. You have to search for it, but finally you can deactivate or modify settings previously only available via registry/policy hacks.
Finally you can uninstall more components, like the Media Center, Media Player or Internet Explorer without any problem. Some things (like CD/DVD burning or the new "Libraries" features) still need registry hacks, but the difference is enormous.
Getting into speed and user experience, it is way way more smooth than Vista. Feels like a Windows XP on a high-end pc, but with all Aero features turned on. This time you really feel the system responsive to every action and not sluggish when "multitasking" with it. File copy operations seem to be faster than XP ones, Office, Visual Studio and other applications "grey windows" (of unresponsiveness) have dissapeared...
Small details have been fixed, like the stupid Vista shutdown button (which put the PC to sleep instead of shutting it down by default) allows a fast and easy customization. 
It is much more customizable, from the new start menu revision (similar to Vista one but with small improvements) to the new taskbar (which you can adapt to the old system, use the new "always group" one, or a hybrid).
You can "pin down" shorcuts and folders into the start menu, and for example folders will get grouped by themselves. 
And there are other small details that improve a lot the experience of using it, like the new taskbar progress bar feedback. 
If you have a touch screen,finally you can start to really use this windows without a mouse (or Tablet PC). I think that all this features were already available on Mac (like window "smart stickness"), for us the Windows users it's a nice improvement.
I haven't tried it with many games (as I said, it is installed on my development laptop), and I don't plan to do it at least until after it is released to the public, because I already suffered the Vista hell of videogames and now that it is stable I don't want to repeat the experience until all major companies support Windows 7 with non-beta drivers or patch their games.
As a conclussion (and critic for Microsoft), Windows 7 is what Windows Vista should have been. It is fast, secure, customizable and really improved from Windows XP.
If Vista didn't existed it would be a great improvement. With Vista on the market, Microsoft should offer Windows 7 as a free upgrade to every Windows Vista owner.