I haven't read all the posts here about how bad 9.1 is but this is my experience:
Installed 9.1 when it first came out over my old 9.04. I didn't notice any improvement or worsening. It was not more crash prone than anything else I've used before. Couldn't recommend it. But couldn't recomment against it either.
About a week ago my hard drive started making strange noises. I decided that I hadn't done a clean install for a year or so and that I should take this opportunity to do clean up. I did a low level reformatting and installed first 9.04 and then the update to 9.1 and after that used Apple's software update and updated with the recommended software. Then I installed all my stuff.
What have I noticed?
1. Extremely stable. No crash since last week (normally one per 48 hours approx.)
2. Generally snappy feeling compared with 9.04
3. VERY FAST wakeup time. Just three seconds as compared to 9.04 which took around 10-15seconds!!! So nice
4. The strange hd noises are totally gone.
I'm extremely pleased with the upgrade. Before I reformatted 9.1 wasn't faster, more stable or faster wakeup. After it's a new machine. Okay, maybe not a new machine but an improved one at least....a pismo 400mhz by the way.