Ok, so the 2.4.0 upgrade froze up on me first time out. I actually blame myself for that one... I was messing around with the device just as it hit 100%. One quick paperclip reset and the 2nd time went smoothly.
Only real problem is that the upgrade messed up my Adobe Digital Editions authorization somehow. Tried logging out and logging in to reset the database, but no joy. And I think I've been bit by the infamous six device limit (with re-auths counting as new devices.) So now I have to talk to Adobe. Oh joy.
A pleasant surprise though... when I logged into my Kobo account it brought me to the Bookstore page (changed from the old Discover page, which I bet some folks found confusing) and it had a list of 180+ books that they had special offers on. Ended up picking up a "Richard Castle" short story for $0.37, so thanks, Kobo! The special offers page disappeared when I went to my Library (it's apparently meant more for first-timers) but for fun I tried logging out and back in yet again. Back came the special offers.
I will say that things seem a little snappier with 2.4 compared to 2.3.1. Haven't had a chance to do much testing though.