I have been using IE7 for about two months. Firefox is my default browser, so I actually don't so much web browsing with IE7, but I use it to see how it displays the web pages that I author. And just like IE6, things are displayed differently then say Firefox and Opera, which display content similarly these days.
It is not an extreme difference by no means, but just last year, Firefox and Opera teams got together and at least tried to make their respective browsers somewhat parallel to eachother in regards to how content is displayed.
Where is IE for all this? Well just as jgweed states, they were to busy trying to play catch up, and I think that this is just a setback for the overall developement of things to come, mainly because of the popularity of their Operating Systems.
I am not trying to bag on Microsoft at all, but sometimes pride does get in the way of growth, at least that is my opinion.
I have found a few annoyances in IE7. The toolbars are very buggy. They don't do what they are told, and the first thing I thought was I got hijacked. So I sscanned with HJT analyzed the log, and all the normal expected entries were there. I still did a couple of scans just in case. Clean as usuall.
Also, it seems to manifest page loading errors frequently, which I discovered that emptying the cache, and reloading the page resolves the issue until the next time it happens. I noticed it started doing this when the cache gets over about 18 - 25 Megabytes. None of my other browsers have this problem(Avant, Maxthon, Slim Browser) and they are all based on the IEshell, so I am guessing it is just one of the bugs in the current beta version.
One last thing I have found by using IE7 is that it practically refuses to download .reg files, which I have configured to be allowed. It's not the settings, nor the host file, or any of my security apps. Once again all the other browsers accomplished the feat with no problem.
Hope this is usefull,