I'm hoping it's going to give me enough proficiency and self confidence to really master some of the synths I use all the time, like Element, Massive, Monark and the like. It doesn't cover everything that's on those synths, but I think it will give me the skills to approach my experimentation in a more intelligent, controlled manner - instead of twiddling without really understanding what's going on. It's like I've learned the basics before, but never fully made the leap from theoretical to practical. Syntorial seems like the ideal way to bridge that gap.
Of course what it's never going to do is teach me how to use highly complicated and unique synths like Prism, which no matter how many times I read the manual I'll never understand until I have a degree in digital audio (and probably physics & math). Although given that I can't find one single complete tutorial for that synth except the short overview video that Native Instruments put out, it seems that I'm not alone on that score
Another very helpful course is Sound on Sound's "Synth Secrets" series of articles. It's absolutely huge and goes into the nitty gritty of synthesis (including the math & other advanced synth topics) in great detail. I got about 25 articles into the series before stopping for some reason, and I've just reminded myself that I really need to start reading those again. You can find the first part here:
http://www.soundonsound.com/sos/may99/articles/synthsec.htm and just Google for part 2, part 3 etc....