I've been using Win7 RTM on both my desktop (a fairly powerful quad-core system) and my netbook (Asus EEE 1000HA) since about a week after it's final build, and I've got to say it's fantastic. Others have already mentioned some of what I'm about to say, but I figured I'd condense it into a list.
Hardware support - this is fairly key, since scalability with newer systems is something that is done better by Win7. XP's multi-core capabilities are starting to show some lacking, while Win7 was built with multi-tasking and 64-bit in mind. Furthermore, with the EOL of XP coming up within a matter of weeks, hardware manufacturers will be hard pressed to find reasons for supporting a dead operating system. Much more hardware is supported and functional from a fresh install, allowing you to at least use the system without it being half-crippled. I reinstalled Win7 Professional on my netbook tonight (it had Ultimate on it, but I had no license, and activation period ran out), and I was able to sign onto the wireless network, download all the drivers, and install them. What's even better - sleep and hibernate were available out of the box!
Hardware support (part 2) - many power saving features have been greatly improved in Win7. One feature in particular (core parking) allows current systems to save power far better than previously possible. Specifically, it lets the system "turn off" cores of a CPU in order to save power. If you're simply using your laptop for word processing or watching a video (something that doesn't require all cores), you can extend your battery life even further.
Usability. Anyone who says that XP is more user-friendly than Win7 simply isn't willing to change. Time and time again, I've introduced family, friends, and random strangers at coffee shops to the way things are setup in Win7 - and every time their response has been "why wasn't it that way in the first place?" Breadcrumb layout in explorer (a feature that was in Vista as well), improved wireless accessibility, simplified and cleaned up start menu, single-icon super bar (something that's wonderful on a laptop/netbook) - the list goes on. I had XP on this netbook for about 6 months, and the moment you ended up with 8 or more windows (something that's very easy to do with IM programs) - good luck finding them in the taskbar. The moment you started installing your commonly used programs - the horribly fanned-out XP start menu would start filling up your screen.
Security - this is one place where things have VASTLY improved in comparison to XP. UAC prompts are simplified and less intrusive, meaning that fewer users will mindlessly click "yes". I could go on for hours about this, but in short Win7 is miles ahead of the near 10-year old operating system.
Libraries - Oh my gosh, libraries. Where would I be without you? The moment you start using multiple locations to store data, or multiple hard drives - these libraries become invaluable. Consolidated data along with built-in indexed search means you get to the files you want faster. Anyone who says that it requires "lots of resources" or "slows down the system" should play with my netbook for a while. Start to finish - you get your work done faster.
Improvement of the built-in tools - Paint got a pretty significant facelift, and I've got to say, I like it. The built-in ISO burner has worked just fine for me so far without any significant hiccups. The Snipping Tool has become pretty invaluable for me recently. Want to send someone a screenshot? Click-click - done. Desktop gadget performance has improved as well - something that used to bog down some Vista systems.
Multi-monitor support and visual effects - I've got a three-monitor setup at home (yes, three), and in terms of programs showing up where they should, window management using Aero snapping, and desktop shortcuts - Win7 is a clear winner again. Flip3D performance has been improved. Win+Up/Left/Right/Down is such a simple keyboard shortcut for window placement and organization. Aero Peek (what happens when you alt-tab and pause over an item) and the new location for the "Show Desktop" button have also been refined.
Windows Media Center - this isn't something that I've played with too much, but after using the XP and Vista Media Centers, the layout and responsiveness are both better IMO.
Now that my rant about my list of improvements is done, I'll go onto another point that many seem to have forgotten over the past 9 years - money. The cost of building an up-to-date system that is capable of handling most day-to-day tasks is by far cheaper today than it was when XP was released. Windows7 version pricing is less than XP was (Win7 Ultimate is cheaper dollar for dollar than XP Professional was), and building a system that's capable of running Windows7 with all the bells and whistles is significantly cheaper than it was in 2001. Are you going to be able to run Win7 with your 9 year-old computer? no. Will you get more out of your computer if you spend ~$500 to build a new one? Most likely.