Defensive Computing is the theme here and this means never jumping on new versions or releases of software (not to be confused with patches to existing software).
New versions are inevitably buggy and suffer from incompatibilities that haven't yet been discovered and/or documented. There may also be small quirks and it takes time for others to discover them and document work-arounds. Waiting also lets others come up with tricks or improvements or add-ons.
If your computer is not used for anything particularly important, basically a toy, then go for it. But this blog is for people who depend on their computers in a non-trivial way.
The risk/reward tradeoff pre-supposes a reward. So, back to Internet Explorer 8, does it bring a lot to the table?
One new IE8 feature is private browsing (actually three different features). Anyone interested in web browsing that leaves behind no traces should look into Sandboxie, which works with any web browser. Or, use the portable version of Firefox available at portableapps.com.
Internet Explorer 8 has made significant changes to the way it renders web pages. Webmasters may have a long adjustment period before they get up to speed on the changes.
The risk of installing new software is always mitigated by the fact that it can be un-installed. But falling back from IE8 to IE7 is, in my opinion, unusually risky since Internet Explorer is so much a part of the operating system.
I'm sure the fallback works and Microsoft tested it, but it's a bigger, more complicated deal than uninstalling a normal application, one that's at arms length with Windows itself. I suggest making a disk image backup before installing IE8.
Firefox users have it easy, just ignore IE8. I personally didn't start installing IE7 until it had been available for well over a year. For the most part, I only use Internet Explorer to run Windows Update and the occasional ActiveX utility.
Even if you're an IE user, you should have Firefox installed before upgrading to IE8. It can't hurt.
My gut feel for IE8 is to wait at least a few months. More if your computer is very important to you and still more if you don't know how to make a disk image backup.
Being in the "ask me a question" business, he's in a great position to judge how IE8 has been treating people. For many, he reports, IE8 broke something. The reader comments are also an interesting read.
Waiting is the way to go.
Update: June 11, 2009. Saw this coming: IE 8 causes big problems on some PCs
Update June 12, 2009. And this too IE gets 'fixed' and users get panicky