Ok, I've upgraded to iOS 5.1, and here's my 2 cents.
1) The performance of the Music app has been improved. On the iPad 1 it's definitely smoother, more stable and more responsive.
2) Most notably, the return of 0.5x and 2.0x speed control and 30 sec rewind is great.
3) Audio from the iPad speaker seems much cleaner and louder - very nice for portable podcast listening.
4) I'm finding the linear scrubber control behavior to be much smoother, whereas before the action was terribly rough and jittery.
5) Selecting playlist, artists, songs podcast etc. seems just all 'round snappier.
6) The overall OS performance with the iPad 1 seems more stable and smoother (although it still has its bugs, I think iOS 4 was better for the iPad 1).
Now, what I don't like about the Music app:
1) Playlists and Artists still show up as an x-y grid with pictures of albums. I HATE this with a vengeance. To me, it makes it makes no sense, it's virtually unreadable, visually cluttered and horrible to navigate. Whoever thought of this model for navigation, imho has poor judgment in data presentation. To me, it's like designing a phone book with names running left to right in regular reading pattern - it's just stupid and illogical - and this is exactly what the Music app does! I would love it if they gave us the option to sort the lists vertically, or with some type of vertical branching.
Fox!