I did Katahdin last summer (July).
(1) We had planned on the Knife's Edge, but it had been raining non-stop for a few days when we arrived. And the ranger was brutally honest that the danger level on Knife's Edge wasn't worth the risk. Done and done. We looped up from Roaring Brook camp via Chimney Pond and the Saddle trail, then to the summit. Back tracked a bit down the (actual) saddle over to Hamlin peak, and came back down the Hamlin trail. It was hard, and a blast.
(2) Scenery is gorgeous, make sure to have a camera and a big memory card. For us, the summit on Katahdin was clouded out, so our pictures on the summit lose a bit of the awe factor.
(3) Bugs. Nats. Flies. Dear god, this was the worst part. They were everywhere. I think they're called "Black Flies" and they were covering us head to toe at the tree line and below. On our early morning start (around 0400) it was fine, but coming back down around lunch time/early afternoon it was miserable with the bugs. If you're sweaty they are even worse. If you can bring any sort of spray or repellant, do it.
(4) We stayed the night at Roaring Brook- so reserved a slot for Friday. Got there Friday late in the day and setup camp. Woke up early, packed up and moved our car about 90 feet to the day lot, then headed out for the trail. This made it easy to get a camp spot (since you're reserving for Friday and not Saturday).