I have a Nexus 5 and am using the stock Android encryption.
I very rarely have to boot up my phone, so I don't know if it made the boot up time any longer or not. Seems fine to me. During boot up though I do need to enter the encryption password and this is the only time that's required.
Background activity such as music playing work fine with the screen locked.
When I plug the phone into a computer, I see the phone in Windows Explorer, but nothing shows up unless I unlock the phone. When the phone is unlocked and plugged in, then I'm able to access the files from the PC. And by the way, if the phone locks after I've opened the Phone in Windows Explorer, I'm still able to see the files.
So encrypting the phone has been pretty painless for me in my everyday use of the phone.