Jon Box, I'm using the IE8 Beta everyday on my main machine. It works reasonably well. While I look forward to when the next releases come out, my experience would be pretty satisfactory except for 2 issues typically - sites that incorrectly detect my browser version, and sites that are applying IE7 CSS rules against my browser that has CSS 2.1 compliance.
Incorrectly detecting browser versions, or User-Agent Sniffing
There are a couple of ways to get this incorrect, and the basic issue is that some sites did not code for a future version past IE7, which is understandable. Even internally, we have some sites that require modification because the version check does not fall within the coded range of IE5-IE7. Just note that in this version, IE8 is emitting "MSIE 8.0" in the user-agent string, and it's a frustrating thing to get a message that says that this new browser version is not supported on this site, and to go download IE7. Very easy to fix though.
Applying IE7 Rules against IE8, a CSS 2.1 Compliant Browser: As I mentioned this is the other issue that I'm having, and it deals with a change that we are making. To understand this, there is a little background required to explain the scenario.