[sciurius]

Maybe I misunderstood...

I have a song in C. The transpose dialog reads C/Am and has Sharp enabled.

Pressing ">" a number of times gives the sequence C# -> D -> E -> F -> F# -> G -> A -> B -> C.
D# and G# are skipped.

With Flat enabled I do get the complete sequence:
C -> Db -> D -> Eb -> E -> F -> Gb -> G -> Ab -> A -> Bb -> B -> C.

Another song is in Em (G). The transpose dialog says that the original key was Gm. Where did the Gm come from? Apparently, the song was originally entered with Gm in the key field. So I changed this to Em using the Edit dialog.
To my surprise, the Transpose dialog still insists that the original key was Gm.
The chords of the song are Em C D Bm. Transposing one step up (well, two steps, G -> A, see above) with Sharp enabled gives Gbm D E Dbm.
With Flat enabled, one step up gives Fm C# D# Cm.

Summarizing, the new Transpose dialog is great and much more intuitive than the old one. Personally I would prefer the buttons to go up/down instead of right/left.
Some quirks: D# and G# are missing from the Sharp range, and it seems that the role of Sharp / Flat is reversed when the key is minor.