Loaded this on my wife's Eris also. Seems to work just fine. Phone seems a little faster, but still nothing like a Droid.
Concerning Flash, yes the previous Eris OS had a version of Flash lite.
Some observations of this version of 2.1 as compared to the "normal" 2.1
The 2.1 3D Gallery is not included but does install and work OK (wouldn't before with the 1.5 OS version).
The 2.1 News and Weather widget is also not included but does install and work OK (wouldn't before with the 1.5 OS version)
The 2.1 Launcher is also not included and will install BUT when you try to use it it goes into and endless FC loop.
Of note, when I installed this it gave me the option to use the "stock Home" and it is the old version with only 3 screens and the normal looking 2.0.1 app drawer. But it is interesting to turn off the Sense home and use the stock home for a little while. I preferred that to the Sense home, but my wife didn't so I switched it back
As exprected when you turn off the Sense home the phone is MUCH faster on the native Home.