1. Measures up about the same, overall. Motion 5 does not seem to be missing any features over 4, but things have changed, like roundtripping to FCP.
2. Rigging, and compound effect creation for use in FCPX.
3. Sometimes seems more sluggish than Motion 4. The new interface is good and bad. Color scheme is nice, but you cannot break apart the interface and use on multiple monitiors as easy as before. I have a cinema display and another dell turned portrait for panels and directory windows and whatnot, and I can't peel them away from Motion 5 (or can i?).
4. If you are going to use FCPX, then the upgrade is worth it. If not, then the upgrade may not be worth it. Motion 4 works just as well in Lion (so far).
Overall, I can create stuff faster in Motion than in AE 5 (which is also very nice).