Angela S.wrote a review of Hydro Photon SteriPEN FitsAll Filter on February 1, 2013
Familiarity: I've used it several times
So basically I use this and iodine tabs to get my water on the trail, this does a nice job of filtering out any pieces from the water. The screen flows rather slow though, and it the cone does not fit all that well into the Platypus brand water bottles (if you can call those bottles). The two pieces of the cone are hard to get together.
My son has one too, and he uses the Steri-Pen with it in a Nalgene bottle and it fits tight in the bottle and the pen fits tight in the cone.
Overall, it could be better, but it does the job and it takes up only a small amount of room in a pack.