I was going to write a preview of it, but blogger SF Mike has a post that captures the enthusiasm for the upcoming concerts as well as can be done. You can read it here.
On the other hand, Josh Kosman of the SF Chronicle brings up some valid points about what (and who) is missing this time around (or how much 2012 seems like 2000 all over again). You can read that here.
Having missed it before (I was traveling that summer), I'm more in Mike's camp, enthusiasm-wise. Regardless, everyone admits it's a big deal, including Alex Ross of the New Yorker, so if you don't want to miss one of the major music events of the year, get a ticket now before they're gone. The Symphony is offering an incredible deal- $100 will buy you a pass that's good for every concert. Mike says they're going fast.