Agreed on the late night comment. Most of it is a long drive back dirt roads, plus a long walk-in, and at this time of year so much happens right at dark. Take a reliable flashlight, expect not to get back to civilization till 10:30 or 11 p.m., and have fun with it. That's half the fun of Penns....
Also, if you're used to places like Spring Creek and one fishermen per pool, well, realize that a "pool" in that section of Penns can push a mile long, and the riffles can be just as long. In terms of width, it's big, but not huge water. But the whole scale of the riffle to pool thing requires a true mental adjustment.
At Green Drake time, it's TRULY crowded, by any standard. At any other times, if you get away from access points it's not all that crowded in terms of fishermen per length of water, you'll have plenty of elbow room. But in terms of fishermen per pool, well, aside from mid-winter it's hard to expect a 3/4 mile long pool to yourself.