I took the Fenix torch out for the first time tonight, waited until it was pretty much dark at about 6.30 and set off on a 20 miler. The first 10 mins or so I was playing around with it to get it to sit nicely and point where I wanted it. Found the sweet spot and from there until the end it was no trouble whatsoever. By the time I was home I was happy to take it off but that could easily be down to me having it done up too tight. I'll play around with that more in time. Overall though the fit was incredibly stable, it never felt too tight, it never felt like it might fall off, it didn't bounce around at all. Having the top band over the head really helps I think.
As for the light it game off, I was very happy with it. I found myself flitting between the 50 and 120 lumen levels. On country lanes in the middle of nowhere (i.e. dark) 50 was fine, on even trails 50 was fine most of the time. When I got onto the Ridgeway it was quite slippy and the ground isn't even so for most of that I used it on 120 and again it was fine. Takes a little bit of getting used to but that will be the same with any torch. I never once felt like I didn't have sufficient light, or that I had to run slower than normal because it was dark. On bits where you didn't have to watch your footing quite so much 50 was usually just about ok too.
Scariest bit was going up a really narrow road with trees either side and over the top, owls calling out constantly and thinking of the Blair Witch Project! Then later on I suddenly noticed two pairs of eyes looking at me, so freaky! Overall though it was a great run, very liberating knowing that just because it's dark it doesn't mean you can't keep running.
I would definitely recommend the Fenix light. I know I've not used the Lenser so can't directly compare, but those were the two I was choosing between. The biggest thing going for the Lenser for me was the focus system, I thought I might miss that on the Fenix but really, you don't need to adjust anything. Having a regulated supply and decent waterproofing imo are much more userful features.