What Tracy E said.
Motion 5 appears to be slower than Mo4. If you don't have FCX, there is little reason to upgrade as you won't be able to "round trip" as you can with FCP and Mo4. Also Motion 5 can only open 1 project at a time, you don't get Finder "Quickview", you can't open the Motion projects in Quicktime, certain plugins (that I found useful) are no longer available, you don't have support for external monitoring or digital cinema desktop, and you can't break your user interface up and spread it across multiple monitors.
That said, if you have FCX, then you really should have Motion 5 as it is the effects interface for FCX.
Finally, it really isn't an "upgrade" per se... if you purchase Motion 5, you still have access to Motion 4. However, unless you have Motion 4 already running, FCP's "Send to Motion..." will incorrectly invoke Motion 5.
Hope that's the info you're looking for.