Does not have to be rooted.
It's not a quick process - it had to update hundreds of apps, and when it finally finished, the phone was so hot it got sluggish and videos wouldn't record. Took almost 30 minutes.
I didn't notice any real improvements other than, for some reason, the old version of Tapatalk, which loaded posts a lot faster and didn't constantly hang.
However, the phone went black and completely locked up on me twice and then got stuck on the Sprint Spark screen when I rebooted, necessitating a battery pull. After the second time, I went back to Dalvik.