I used the Android-provided encryption on my N4 last month. I was feeling a little paranoid about losing my phone and the personal info accessible from the phone. I can't tell it runs any slower. Here's what I absolutely do not like about it:
* The same key is used on start-up and waking from sleep. Why, friggin' why! I had to immediately change my long password to a short one, partly defeating the purpose of encryption. There's no way I'm putting a long password in every time I want to wake the phone up. What are you thinking, developers, when you did this? Are you that lazy and this is all you could come up with?
* I find the start-up keyboard cumbersome to use, again pushing one toward the use of a shorter, less secure key. Maybe I'm spooled by Switfkey.
Can't undo now without a factory refresh. If I had it do over (without losing my phone), I wouldn't do it again. Of course, if I lose my phone, I feel better having done so.