I never used 8.0 and I just start domino development 1.5 years before using 8.5 -> 8.5.1 -> 8.5.2 -> 8.5.3 -> now 9.0.
So far everything is OK with designer 9.0. You can save the hassle of installing the XPages Extension Library because its included in it and there's some new thing like built-in Javascript debugging (haven't really used that though).
The only problem I found so far is the same as
this guy and I don't know what's wrong with my designer. My colleague whom also use designer 9, didn't set any theme to his XPage, but the Date Picker is OK when he preview in Notes. But when I previewed in mine, I got the problem.
Anyway if you'd like to wait some time for more user to use designer 9, its better to upgrade to 8.5.3 with fixpack 4 and extension library. I'd say its getting more stable for XPages development.