The Aperture forums and communities have been rife with upgrade woes for Aperture 3.4. I must've been one of the lucky ones, as my upgrade from Aperture 3.3.2 was smooth - at least as far as my photo  libraries and actually launching the software.

I just found today that my custom keyboard layout was whacked as part of the upgrade:

Notice there isn't one named "Scott" there. I did peek into the ~/Library directory as suggested by this forum post, but found zero. Luckily for me, I could count my customizations on one hand, so it was quicker for me to recreate than restore from a backup.

If you haven't upgraded yet, I'd hold off and let the bugs and gotchas shake out further. I'm quite nervous about the 3rd party Aperture plug-ins not working now too.

UPDATE

Indeed, there are quirks with plug-ins. I can state with confidence the problems described below are caused by Aperture 3.4.

I only use two plug-ins regularly: Photomatix and Perfect Layers. For both, when the plug-in is first launched it's like I never launched the plug-in before. Any preferences are gone, and in the case of Perfect Layers, I got the quick tutorial walkthrough again.

The Perfect Layers plug-in is working as expected. Images make the round trip to Aperture without a problem (at least for the handful of images I've processed since my Aperture upgrade).

The Photomatix plug-in doesn't work. I can tone map, but when trying to save it asks for a license key. I enter my license key, but the plug-in claims it's invalid. The standalone Photomatix works fine. So, I'm not dead in the water, but this means extra steps in workflow which is never good.