Installed IE9 on my Vista Ultimate SP2 x64 machine and what a mess! I used the MS download x64 version without BING. Well let me say there was BING and it took over as the main search provider no matter what I did. The interface was just plain lousy and to top it off some web pages would not open at all, and my machine ran slower than it did on IE8. So I reverted back to IE8 and my machine still behaved erratically, BING took over again
for some reason my machine acted 'jumpy' and slow to respond.
The upshot for me was to System Restore back a few days, reinstall a couple of updates and now all is well. Maybe IE9 works o.k. on Win7 I don't know and I won't even attempt to put it on my laptop with Win7 SP1 x64 as I fear it will 'bork' that too.
As I recall when IE8 came out a lot of websites were not ready (including one of my own banks) and there was an site access problem for sometime until websites were made IE8 compatible so this may also be the case with IE9. All I can say is don't be in a hurry to use IE9.