I'm happy they finally decided to add MP3 support back into it. I've been sticking with the previous version all along.
However, rendering my typical tracks in MP3 with v.10 is still increasing the average file size by 10MB vs. the same track in v.6.0.5 of Garageband.
For the same quality thats technically impossible.
Also GarageBand probably has the same Mp3 codec as LPX, which doesn't make bigger files than LP9.