Does anybody run with their Alpkit on trails, through woodland, etc - rather than just on roads? How does it perform during those runs?
Yes all the time, most of my running is trail. Also use it on 50 mile ultra's with navigation in the dark.
The only issue with head torches (and that is all of them) is that the light is directed down your line of sight. Consequence is no shadows. Often when trail running it is shadows that make objects visible.
Try running with light in your hand - many get sea sick as the light waves around. On your head, no shadows. Take your pick. Two torches one head one waist/hand gives the best of both and does help a lot. A few of the ultra competitors have chest clips, I have seen some for Petzls - the little 4 LED ones. Always in addition to the head torch which is the only one that looks where you do.
I have had three Gammas but got one as soon as they were out so more than happy with the longevity of them. One stopped working, one broke the clip. I never queried Alpkit, for £11/£12 and now £15 they had lasted more than long enough. I run 3-5 times per week in the week for 6-months of the year. I did all my marathon training in the dark. 3-hour long runs on Monday evening. However I am generally 8-10min miling.
If you are really quick then two may be the best option.