The SDcard booting is really handy. The performance is slightly lackluster considering the read speed of the SDcard, but, it's still very usable.
The battery life out of the box, well, I didn't get very far in using it stock. The sales rep claimed 10. I browsed a lot for the better part of an hour without a single notch coming off of the battery gauge, for what that's worth. I know that when I set it to stock speed (300min 800max) I get about 4 hours of non-stop web browsing and all that entails (flash ads, opening multiple windows, checking email, gtalk sessions). Disable WIFI and I'm sure you could double that easily for just reading. Now, when I have it OCd (800min, 1300max) I get 2.5-3 hours of non-step web browsing, etc. The same for game playing, though admittedly I only play scrabble and puzzle games. I have tried a smattering of action games and they are all playable but I can't comment on their battery consumption habits. This is all with screen at max brightness, max volume (and then some thanks to DSP manager).
The Bluetooth is also handy for transferring files, though I haven't tried it for playing music.
I use Cyanogenmod7, though some people swear by Phiremod. MIUI was just released for it...it needs a little work still but it's fully functional, it just doesn't 'scream' yet.
...and as far as any Android hacking is concerned get over to xda-developers.com forums. Nook Color is listed in the forums list. Go straight to the Development sub forum. The boot images are all easily accessible and all the tools you need are right there in a sticky at the top with very clear instructions. You can even dual boot from the eMMC. I have a dual boot bootloader that lets me choose which to boot from eMMC (primary or secondary images), SDcard (also with two images) or Recovery.
I can't say enough good things about this machine, especially when compared to other "tablets" in and round it's price range. That gap is closing in the next 6 months I'd imagine as we get into the next generation of tablets, but, for my needs this thing is fantastic. Good luck.