Go this summer!
Ditto re: bringing 2 liter capacity (per person). Last summer, I completed the JMT in late July-Aug and there were a few stretches during which I was *very* pleased to have my 1 liter bottle + 1 liter collapsible platypus full.
I too am going to do the JMT this summer (same time as last summer) and because I expect some of the tiny streams to be dry this year, I am contemplating a 2 liter collapsible rather than a 1 liter (for a total capacity of 3 liters). I drink a lot though (7-9 liters/day), and I'd rather carry the bit of extra weight from a larger platypus than be limited to only 2 liters (esp. as I'm going alone).
Another poster noted there will be rivers running... yes, where there are rivers they will be running... but there are indeed stretches where there are no rivers. Study your maps, gather info from other hikers as you go - you'll be fine. Hydrate heavily where you can if you know things will likely be dry ahead. As long as you hike smart and do a bit of planning along the way, there's not need to worry.
I'll mention one more thing - last summer we hit Lake Edison (VVR) a few days after the ferry stopped running. Looking at our maps, it was just a ~5 mile hike along the edge of the lake to VVR. No big deal, right!? Wrong. It was miserable. We all emptied our 2 liters, the hike seemed endless, up/down, dry, and not at all close to the water. There's a good chance the ferry will stop running even earlier this summer (particularly if it's a hot one and southern CA needs the energy), so if you end up hiking around the lake, hydrate a ton and fill up before you do so!