I've been playing with the 1284p for about 6 month now. I love it. 16 kb of RAM comes in handy as you basically never have to worry about maxing out and hanging and having to troubleshoot that.
Glad to see the 1284 community growing! I use the Mighty bootloader as well. Maniacbug did a great job. Something to note is that you may want to include this branch with the bootloader so that you can use INPUT_PULLUP: Implement INPUT_PULLUP · wijnen/mighty-1284p@61f24a9 · GitHub
(It's just one or two lines of code, so you can just change it on your own).
I'll post some info on my project soon. For now, I am using the DIP package, but I will soon be in SMD land.
Cheers!