There are a few things you gain with it. For the general Cyanogenmod features, the CM features page does a better job of explaining than I would here: http://www.cyanogenmod.com/about/features
The main A1-specific improvements I can think of off the top of my head are:
- The capacitive buttons light up when you turn the screen on and go off when you turn it off. You don't need to press them before they light up. You get haptic feedback with the buttons as well.
- Overclocking. By default the A1 runs at a max of 1GHz. CM7 allows you to go up to 1.2GHz, but it doesn't overclock by default - you can install an app like SetCPU if you want overclocking.
- Some general performance enhancements.
We're still working on getting USB host mode working so that you can plug in USB disk drives, mice, keyboards etc - we are hoping that it will be possible to enable this functionality.
CM7 is based on Gingerbread - the Android version is 2.3.7, as opposed to the stock A1's 2.3.4.
Gingerbread itself is phone oriented, but CM7 includes a set of extra tablet-specific settings and tweaks.
It is compatible with the Play Store and all the other Google Apps. I see no reason why the Amazon appstore shouldn't also work. I believe you can install the GO Applications from the Play store, but they're not included by default. I did forget to mention earlier that you need to wipe your data partition when installing CM7, which will remove all your apps. However, if you use an app such as Titanium Backup, you can backup all your apps, wipe and install CM7, then reinstall your apps afterwards. Installing CM7 doesn't erase your internal SD card though.
You do have to bear in mind that the A1 port of CM7 isn't yet 100% complete - although most things work, there are one or two kinks to be ironed out yet. You can check out the list of known issues on the XDA Developers thread: http://forum.xda-developers.com/showthread.php?t=1439451
