One thing I noticed about moving from 2 to 3 which wasn't great is if you have any devices set up in iTunes which sync photos from Aperture projects or folders, they get unselected, and you have to set them up again. I had about 30 projects/folders ticked out of 100+ total to sync to Apple TV. After the upgrade, iTunes settings for Apple TV had changed to not sync any photos, and I had to go and select all the ones I wanted to sync again. Of course it then deleted them on Apple TV on the next sync and resent them, even though they hadn't actually changed. That was 15GB of network traffic that wasn't strictly necessary.
Other than that, so far the upgrade has been overall very positive. On the downside, it's crashed on me twice in about 20 hours use, and I'm not convinced it's any faster than 2 was - it seems about the same to me (2.93 UMBP with 4GB RAM).
There are a lot of really good things though. The new full screen functionality is great. The Places stuff is awesome if you have geotagged photos. I have been geotagging for two years even though up until now I had no way of making use of it - now it's all come to life. I imported some GPS tracks too and it's neat to be able to see those on the map as well as the pins marking where you took pictures. And the whole adjustment preset thing is superb - and you can even apply a preset when importing which is one less step afterwards. Now you can export as library, merge libraries and sync libraries I'm wondering about getting a Mac Pro as well. I haven't even got on to a lot of the new things like brushes etc yet. Overall, well worth the upgrade cost for me.
I have a tl;dr crash report. An excerpt:
