I'm really pretty neutral on Internet Explorer 8 right now. I haven't played with it yet. Microsoft just moved IE8 over to "Critical" status so it'll get pushed to a PC near you soon. I'm pretty conservative on letting things get pushed to my PCs.
With IE7 I used the IE7 blocking tool from IntelliAdmin.com. It worked great.
With IE8 Microsoft came out with their own blocking toolkit. Needless to say, it's bigger than a breadbox! It ends up creating a script that you have to run from a command line and set switches. Yuk.
In the end, it simply creates a single registry key. I created a pair of .reg files that do the same thing.
Just download this zip file. In it are 2 files, one to block IE8 and one to unblock IE8. Just open the zip file and double-click on whichever you want to do. Click on "Yes" in the dialog box that comes up and you're done. Look at them pretty closely. Use at your own risk.
As I was preparing this post, I went back to IntelliAdmin.com to see if they had done something for IE8. Needless to say they have. They have added IE8 blocking to their Network Administrator tool. Their article is here.
It's not quite as trivial as my .reg files (it requires an install) but I'm sure it's very good.