I work for the company that provides the official Spring training, so...Yes! It is a worthwhile investment!
Honestly, though, I know that SpringSource training is top-notch. I can't speak for anyone else offering Spring training, though. I'm sure that there are good courses and some that are crap. Whether or not you need the training versus just reading my book depends on how good you are at self-learning and how good the course in question is. There are too many variables in play for me to be able to give you a definite answer.