Well I got the update with a normal modem connection, and the download rate was 4,55 Kbyte/sec. 5 hours !! But the download got okay, and I installed it on both my classic partition and OS9 partition.
With 10.0.4, Classic makes few changes from 9.1. Perhaps a little bit snappier in some window's update, but no big changes. Classic starts up in about 50 seconds, quit the same as previous.
We'll wait for 10.1 to see if 9.2.1 really improves Classic's speed. For common people who don't want to download the software in 5 hours, I recommand to wait for the 10.1 update, which will probably include the update CD for 9.2.1.
In fact, if you didn't download the software updates released since january with the automatic software update, getting the 82 MB update should worth the cost.