Size15's, I have used a lot of other headlamps and this one certainly takes the cake for performance. I took it on a short hike, for a 3 watt it has very good spill and throw. Medium is plenty of light for most applications and high is very nice. The beam is more like a giant spot, on throw, the beam is somehwere in betweeen the Lionheart with the beam difuser and the U2, just not quite as much light as the U2. I plugged the 4C pack in last night and let it run for an hour or so it got fairly warm, the heatsinking seemed to do a good job. It actually has fins on the sink hidden by plastic. Again, the headband is tight and a bit uncomfortable with the AA packs but with the 4C pack on your waist the light is almost weighltess. I am going to take the plastic tensioner off, that should give enough extra band to fit.
grnamin-- The plugs are I believe the same as headphone, maybe a bit small but there is a lip that snaps on, they feel pretty sturdy to me. I don't think you would have any problem jogging with it, especially if you used the 4C pack. I was thinking of making a small battery pack for it with 2 18650 batteries in parallel for my waist, that would be very nice and light.
I haven't found any issues with the connecting battery packs, mine has guides on the band to keep the wires in place.