Update, March 14, 2011: Note that Internet Explorer 8 is now for Windows XP only. If you have Windows Vista or Windows 7, you are strongly encouraged to upgrade to Internet Explorer 9 (download) for improved browser security and better browsing features.
Internet Explorer 8 addresses just about all of the major concerns that users and critics have had with the world's most used browser. Whether they get answered in a way you like is another matter.
There are several new and interesting features. Web Slices lets you save predefined sections of a Web page for at-a-glance viewing. Instead of going to a traffic Web site for updates, the latest commuting news comes to you. Similarly, Accelerators make repetitive tasks one-click behaviors, for instance finding directions or blogging. InPrivate browsing introduces a cache and history on-off switch, while related tabs are color-coded and automatically reorganized as you open them. There's also tab sandboxing, which means that when a tab crashes, IE itself won't, and it even tries to resurrect the page that crashed.
There's a greater emphasis on Web standards and security than before. The SmartScreen and cross-site scripting filters throw up a red warning page when you're about to visit an unsafe site. There's also domain highlighting, which grays out the name of the URL you're looking at except for the domain itself. This sounds simple, but effectively draws attention to spoofed site URLs. There's also a compatibility button so that sites designed specifically for IE 7 and earlier can still be viewed.
IE 8 lacks a default "smart" location bar that many other browsers have, but you can search your history and most visited pages from there. Also, the installation process still requires a reboot--unimpressive, to say the least. Drawbacks aside, there's no reason to not upgrade if you're an old fan of IE, and there's even a few things in IE 8 for new users.