I like it. I have the Tikka Plus 2 with the CORE battery pack.
-I set the main mode to blast all 90 lumens, so I have a very strong light when I need it.
-I set the economy mode to as close to the lowest amount of light as I can, so I can have it last as long as possible outdoors.
After playing around with the settings, I found I liked having the headlamp do those two things perfectly. Very handy. The charge doesn't last quite as long as three AAA batteries, but it's close. I can carry spare batteries in the field and replace the CORE on a whim, though, I am a little worried I might crack something if I try to remove the CORE in the dark. I suggest practicing it once.
Also, you can buy the Tikka 2 Plus at EMS for cheap (31$ for me) and then buy the CORE separately, and it'll cost you less altogether than buying the one that comes with the CORE.