Definity is actually pretty easy to learn. It has a very user friendly interface, compared to other PBX's out there.. I've been working with Definity on converged solutions for the last few years. Just tinkering with them gains you alot of experience. Tinkering is the best way to learn Definity, because their documentation is very lacking..
I must give you the first level of advice though. Its definately a different ballgame and you will be clueless about anything unless you have quite a bit of voice experience.
BuckWeet