"One thing I'd look for in a new shirt is permithrin to keep ticks at bay. If you already have permithrin, you can treat it yourself, but it's somewhat expensive if you don't."
I had mailed such a shirt to myself to start the Sierras on the PCT in '08, but then noticed a post somewhere from someone that talked about getting a rash. Then I heard about someone on-trail who had also got a rash on his back from such a shirt. So I mailed it home, and stuck with a regular shirt. Life was fine without it; a good part of the way thru-hikers deal with bugs is by not taking breaks. They're typically a lot less bothersome if you can just keep moving.
I am NOT positive that the permetherim is/was a problem, but I would pay less attention to folks that have worn such a shirt on a week or less outing to someone who's worn it constantly for weeks or months on end --- if you can find someone like that to talk to. Perhaps it also has to do with individual body chemistry too, dunno.
In any event, for long distance backpacking I'm now un-interested in this approach. I still have the shirt, still unused; maybe I'll remember to wear it on a shorter buggy trip sometime.