We dropped 9.1 because the user interface took 3 to 15 seconds to process a mouse click on intel P4 3.4ghz machines, and about the same amount on Core 2 duo's.
In addition, if you changed .mhs files, and clicked to compile, it would tell you the mhs file changed, would you like to reload it? If you said yes, it would blow away your changes. I had this happen even though I had clicked "save" on mine.
The slowness (especially compared to 8.2) and the scare event of changing files we had saved caused us to go back to 8.2. The file change and slowness happened to more than one engineer, so even if it was a "stupid user trick," it happened to several of us, and the "stupid users" banded together and went back a revision.