Xewton is awesome. When it first came out people likened it to GarageBand on the iPhone, and in some ways it is. Personally I like to use it for composing rather than ‘sequencing’ if you know what I mean.
I also love the fact that when I export MIDI files from Symphony Pro I can open them in Music Studio and start messing around with them.
Although I can get lost at times editing in Music Studio it is still an app that I go back to time and again.