Most important to me is that the transposer doesn't immediately "transpose" after I hit the +/- button, but waits till I play a new chord or key before the change. My Yamaha Tyros4 does this with ease, and I suspect the other manufacturers have similar setups.

I usually go through my home-made restaurant fake books and because a lot of the music is written in the original key, I get to play in many keys other than "C", which is good for keeping my skills up.

As far as an "easiest" soloing key, I would have to say "C, "G" "A", or "E" the latter two I became accustomed to playing in due to being in bands with guitar players.

The transposer is especially cool to use when tunes have a lot of repetition.

Ian