I'm torn here, because I often use my laptop (or print-outs therefrom) to make fairly extensive notes on my upcoming sessions. I like to have a lot prepared, so that the players have some chances to veer from the anticipated course. I even recently took up the habit of writing them in Scrivener, because I could keep them organized in little nodes that way, which is how many games end up going.
While I'd love to see this integrated into Roll20, also feel like maybe that is something best kept on my own computer/next to my computer instead. I've tried using the GM layer to put in some notes for on-the-fly reference, but it wasn't suitable at all.
Part of me would love to have a Comments function similar to what you can find in Word or in some PDF viewers. Then a token or even a grid square/hex could be tagged with a pop-up-able comment that has the stats for a trap, or an NPCs simplified stat block. On the other hand, I often have a few pages of typed notes and stats at hand that would be great in a form much like the handouts, but not designed so much for images, nor sharable to the players, but more for text uploads, and for GM eyes only.
I'm curious how other GMs prepare their campaigns, and how that will change (or not) by using Roll20.