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.
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.