It seems fitting that Microsoft Internet Explorer (MSIE) comes in the middle of the pack of browsers: that's where MS likes its software to be. Not quite cutting edge, but filled with features for the majority of users (especially users who aren't tech-savvy).
The big advantage of MSIE is simple: it works with all web pages. Because it's been the de facto standard browser for so long, web pages are designed with it in mind. Some services (especially those designed by Microsoft) only run correctly on MSIE (take a look at Siena's Outlook Web Access on MSIE vs. other browsers). It also does the best job of rendering pages; no oddball shaped boxes as the pages are downloading.
The disadvantage of MSIE is just as simple: security. Internet Explorer has had security problems in the past (as have all other browsers), and the design of Active-X made spyware possible. But the dangers of this have been overstated. Many of the security holes were purely theoretical (you often saw the phase, "Websites can craft a code that") and were unlikely to be a problem if you stuck with legitimate websites. As for the spyware, the security enhancements in Service Pack 2 (back for IE6), helped cut that off.
Still IE7 is a slight disappointment. I expected better: Microsoft made its reputation by not just stealing elements from other programs, but by always improving on them. Yet Microsoft's implementation of tabbed browsing is inferior to Firefox. The blank tab button is a good idea, but it takes too long to display. Lots of times I will click on the tab button enter a web address, and get to the new site on the original tab before the new tab is ready (I've turned off a default page for the new tabs; I use a blank page, and that's still slow).
I also don't like what happens when you have multiple tabs and you click on a link. It opens the link in a new tab to the right. That's standard, but when you close the tab, MSIE goes to the tab immediately to the left of the closed tab, not the originating tab (as Firefox and Opera do).
I also don't like the location of the home, and print buttons in IE7. Microsoft always pushed for standardization, and having them on the tab bar goes against this.
Overall, however, the ability to render web pages correctly overrules the minor issues with tabs. IE7 is a good, solid browser, and, for anyone running Windows, an essential element, as backup to your primary web browser, at least.