[Acer Iconia A500] Neoprimal:
The Good:

Everything is more responsive, period. Moving around is quicker, switching home screens is quicker, moving through apps and widgets is quicker. Not that the A500 was slow before, but it had its hiccups. This gives the tablet new life, imo - it really feels like a different piece of hardware.

The interface is more streamlined. I've never been into form over function, but in this case everything works well together. Many apps now written for ICS fit in with the whole scheme of things which creates a more uniform experience and aesthetic. Many ICS App widgets now allow you to choose either their default or black, which fits in with the ICS color scheme.

Some, dare I say most? video issues seem to have been addressed as I can play files I'd needed to use 3rd party apps to play on the included player now. Flash also seems more fluid.

Notifications are easily closed (with one X vs. many).

Widgets are laid out in a drawer vs. scroll bar making it much better and easier (for some) to go through and add them to screens.

Settings have been improved; for example, there's now a setting to see the data apps use, factory reset is now under a more obvious tab and other settings are found more easily as well.

Booting is fairly quick vs. Honeycomb. Seems to boot in under a minute no matter how many apps are installed. Booting is generally fast based on the amt. of apps that are installed, but I seemed to boot in roughly the same time before installing apps as after installing over 200 of them.

Some of the new features are nice, the screenshot ability is definitely welcome and swiping away apps is also great. Note: some apps will never fully close as they use services that run in the background.

The Bad:

Some users preferred the long press, split screen and 1-click adding of apps and widgets to the home screen vs. the App Drawer approach. I am inclined to agree that this method can be faster for adding multiple app icons and widgets to the home screens.

The Ugly: (bugs?)

The native browser still doesn't seem to be able to handle encrypted content from Google Chrome, and so the native browser cannot be synced with a Google Account which has had its content encrypted. It can only handle accounts with their passwords encrypted (which is the default).

How I see it is, if Chrome Beta can handle it, I don't see why there wasn't a setting added to the native ICS Browser to allow for it. To me, this doesn't make sense. Perhaps I'm just missing something.

***At "excellent" strength and a 75Mbps link rate, upon checking for updates I get a message saying "network coverage is poor, move to a location with better coverage to continue". Hopefully some sort of fix comes from Acer directly before they issue more updates as I can now essentially, not receive updates (apparently).

Will append to list as I come across stuff.

***Update: This doesn't seem to be specific to ICS. After further investigation it seems this WAS caused by something on Acer's end (as of 4/29/2012, updated 5/22/2012).

UPDATE:

As of 5/22/2012 I have not been having/seeing any issues. I have noticed that when the tablet is extremely low on power (under 10% or so) it turns the wifi off when sleeping, even though I've set it to never turn wifi off. I'm not certain if this is a bug or working as intended in ICS as some power saving feature. This never, ever happens when the tablet is on A/C power or docked. When I wake the tablet, wifi is connected to automatically. I'm personally happy that it does this, but I can see it being a problem for some people...so I don't really want to file this under bug/issues.