I have been using Microsoft Office for about 25 years, and have used just about every version. My special area of interest is Microsoft Word, which I have written with and programmed professionally for a living for most of that time.
If you have used Office 2007, the look of Office 2010 applications will be familiar. The most obvious change is that the Office button has gone, replaced by a new File tab just left of the Home tab. The File tab is where you'll find many document-related features, such as information (properties), recent files list, new document creation, sharing, help, and of course printing. All these features have been improved, though none more than the print feature, which has had a major overhaul. It is now easier to use. Its new layout has plenty of space to explain options, and it offers one of the best print-previews I have ever seen outside a professional DTP package.
The ribbon may now be customized. This takes a bit of fiddling, but it is quite easy if you take a moment to watch one of the tutorials on Microsoft's website. This enables you to add any command you like to the ribbon, including macros.
Some of the new text effects in Word are cool-looking but not especially useful to commercial authors. For example, reflections under headings look nice, but nobody would seriously entertain using them in a business presentation or a user guide. However, they do offer some fun design options for informal documents.
PDF-making is now built-in. It used to be available only as a free download (due to a spat between Microsoft and Adobe). The PDFs it makes are acceptable but lack the quality and features of Adobe PDF offerings. That said, you may improve your PDF output by installing a free package such as Nitro PDF, which makes better quality PDFs
The inclusion of OneNote is a plus. OneNote has been overlooked my many users, but it is a powerful note-making tool. I use it to jot ideas for projects. The ability to drop any file into a OneNote page makes it a handy place to dump pictures, video, audio comments, documents and website links for reference.
My only gripe is that, yet again, almost nothing has been added to VBA - the macro programming language built-in to Office applications. Most people don't use it, but it is the most powerful feature in Office. With it, you can make Word, Excel and so on, do just about anything you want it to do. If you can master VBA, then you will have absolute control over Office and the things you create using it.
We've installed Office 2010 on a couple of Windows 7 laptops, and an old Pentium M-based Windows XP laptop. All were fresh installations, and all worked fine. I've read many reviews here that have complained about the terrible installation process, but I had no problems at all. Putting software and hardware clashes aside, the trick is to choose CUSTOM installation (there's an option for this after you enter your licence code). That way you can choose NOT to install the demos of Microsoft Access and Microsoft Outlook. I am startled at how responsive it is in XP, with such a low-powered CPU. It installs in about 1.5GB, which is not bad these days.
Once you have installed it, I recommend clicking the "Continue Online" button to download and install all the updates (there are quite a few). These patch a few problems and generally bring your installation up to date.
Overall it's a pleasing step-up from Office 2007, especially with regard to the new File tab. The new features are not earth-shattering, but I have found them useful and worth the money. For about £25 per licence, it's a relatively inexpensive way to stay up to date with Microsoft Office.