Based on Cheyenne's reassurances that most things run fine, I broke down and installed Ermine on my EE, tried it out, then updated my PE, too. I gave each a run through of all the apps I have installed, etc. Here's what I found:
- the beta appears to be much more stable than I thought
- it runs very fast on both devices - quite an achievement given enTourage's early indications that speed was a real problem initially
- after the first boot on both devices, I got a message about failing to import the library and to use Menu - Library Scan if necessary. Could not find "Menu - Library Scan" everywhere, but all my content and tags seemed okay.
- the browser does seem to crash frequently, but going back to the same page a second time often seems to work (I use Opera most of the time, so not a big deal)
- landscape eInk works, although the eInk refresh is a real mess (for me), require pressing "Refresh" when first rotating the screen
- PDFs actually seem to look worse (less crisp with lower contrast) than before on both devices. Not sure if it's my eyes playing tricks. EPUBs show just fine.
- the PE has Settings - Power Control, including hibernation. The EE lacks this setting and apparently either defaults to hibernate, or uses the older Sleep mode
- Winamp for Android runs great on Android 2.2.1! (Yay!)
- every app I tried seemed to work fine
- if an app came up in a blank window (some people report a "Force Close"), Cheyenne's tip of going into Settings - Applications and using the "Clear Data" option on the app did in fact restore the app to full working condition.
- desktop widgets on the PE seemed to have been undisturbed
- about half my desktop widgets on the EE disappeared, and the grid spacing is now "huge". Must be the screen size.
- I haven't seen any hibernation issues. The EE "sleeps" and wakes up really fast. The PE "hibernates" equally instantly and wakes up just as fast (note you can force the PE to hibernate by long-pressing the Rotate button). Not sure if the power-saving modes are actually going into effect. Will find out more by tomorrow afternoon.

Having played with both for about an hour, I'd say the Ermine experience on the PE is terrific, and only slightly less so on the EE. A wee bit quirky on both machines, but stuff I can live with given the advantages of Android 2.2. I don't think I'll need to revert to the previous version at this point.

EDIT: Turns out the EE does hibernate after a while when closed, but you can't control the interval or force it immediately like you can on the PE. On both devices, the eInk side will show "The eDGE is hibernating" when in fact they are. Pressing the Menu button wakes both up quite quickly.