I've done Allen twice as a day hike. It's one of my favorites. (Number 3 on my Forty-Sixer three-favorites list.) There are excellent views east, toward the Dix Range, and North into Panther Gorge.
As you approach the top, there's a good look at Redfield. I'm drawing a blank on the exact location.
The first time I was on Allen, somebody showed us a good look west toward the then-recently cleared blowdown area, but when Susan and I were up there a year-and-a-half ago I couldn't figure out how to get to that vantage point. I recall that it was somewhat precarious anyway, as we had to balance on branches, so I don't regret not finding it the second time.
You definitely can't camp at the old Twin Brook lean-to site. That's where there's a big, oversized sign with ALLEN > painted on it, pointing to the right (it's about 90 minutes to reach that pont, if I remember correctly, maybe five miles in?), and below the ALLEN marker is a sign warning that beyond that point is private property, so technically the old Twin Brook Trail is closed at that end, too.
Before I actually hiked Allen, I was expecting to have to purchase a GPS unit and night vision goggles, but it's really not that bad a hike. As I said, I quite enjoy it, and I'm looking forward to tackling it a third time with my son. Just pick a nice day, familiarize yourself with the route so you don't waste time trying to figure out what to do next, stay on the marked trail, stay off the private roads (except for the marked stretch that is open to us), and have a great time.
Incidentally, both times I've done it were in September, once at the end of the month, on the 29th, but I still made it back to my car in daylight. (I started at seven a.m., after having slept at a motel in Schroon Lake.) In fact, on the way out, it can be pretty hot in the former blowdown area, because there's no relief from the sun for that stretch, so bring a hat. And if there's a frost in the morning, mind the floating bridge across Lake Jimmy, because it's canted and the boards are slick, particularly when icy. The Allen slide is slippery, too, no matter how dry the rocks look.
I think it probably took me about 11 hours when I did it by myself [Note: Just checked my notes, and it was 11 hours one minute.], 12 when I was with Susan [Close again — 11:55.]. PinPin's done it in under six hours, but I wouldn't expect anyone to match that — at least, not the first attempt.