(my comments apply to the studio bundle - some features may not be available in smaller configurations)
I've put v10 through its paces over the course of a three-week all-night build/animate/render session for a small contest. I'm really impressed. Though there were several crashes (usually related to extreme angles while editing objects, and i'm attributing them to my graphics cards), all but once the built-in recovery function managed to save the scene file (see the "debug" folder by the side of the apps' location) before it went boom.

My personal favourites:
The new "visual selector" tag. A real time-saver. Especially with characters, dragging the bones you need to pick often into it, creating a "switchboard" of click-and-it's-selected items with a basic human shape in the background is bliss. The only way this could be better would be to be able to actually grab the objects on a touchscreen :)
The new timeline. Takes some time getting used to, but after that, it's easier than the old one.
The new elements browser.
Muscles (part of Character module)

Not too happy with:
- the new joints. They're a nice addition, but don't work with my personal preferences for rigging and animating. i.e. they can't influence meshes that contain mirror elements, because they calculate their influence from weighting maps (which the mirrored object seems to miss). The older bones system is still available, so no problem here.
- the new IK system. Fast, stable, great. But doesn't play nice with e.g. hip-to-feet chains. Feets keep rotating in sync with the hip axis. May be my fault, but with "legacy" MoccaIK and target vectors, they don't. So, again, nice to have, but doesn't fit my workflow.
- the decision to turn the manual into HTML help.

All in all, it's an excellent update that i'm very, very happy with. Here's hoping on the next maintenance update to smooth the edges.