I have been evaluating the IE9 beta since it came out in September, and to my surprise it is actually pretty good. Microsoft has had its hands full with IE for so many years fixing bugs and supporting older standards that it is actually pretty amazing to see a product release from them that pushes forward.
When IE7 came out it was remarkable because it head been so many years since we had seen a release from Microsoft. As developers we had suffered for so long trying to work around IE6's many, many bugs. So when IE7 finally came out and fixed some of those bugs we were very excited. Unfortunately it didn't fix all of the bugs from IE6, and it introduced a few of its own. As time went on we came to realize that the uptake of this new browser was also low. Many users still clung to IE6 for whatever reason. So now we had two squirrely browsers to test for instead of one.
When IE8 finally came out, we found this to be a big step forward. It pretty much rendered all HTML4 and CSS2 properly. It was still well behind the other browsers which were on to HTML5 and CSS3 by this time, and it still didn't support many long standing Java Script standards, but at least it wasn't crazily buggy. Finally we had something we could work with. Unfortunately like IE7 the adoption rates were low with many users still using IE6 and IE7. Now we had three different browsers from MS that we had to work with each of which with the potential to render a page completely differently than the others. This while competing browsers all render pages consistently year after year version after version.