I use Sibelius, mixed with Garritan Personal Orchestra, and eastwest Symphonic orchestra/choirs.
I don't want a sequencer for the Wii, but I just love the idea of controlling the tempo with my arms. Simple as that really. Would be nice if Wii Music could import MIDI files rather than be limited to whatever songs are included on the disc. I have some great MIDIs of Beethoven symphonies and the like, and would love to conduct them, even if only on a computer