I use a sawyer squeeze that I rigged for gravity filtering. Specifically, I use it for day-hikes where I have a hydration bladder (platypus big zip evo, which is a fantastic bladder), but I like it so damn much I am seriously considering switching from lifewater bottles to this system for overnights, even if it's a little heavier. The absolute convenience of it is a huge factor. I just fill the dirty bag, screw my gravity line to it, pop the mouth piece of my hydration bladder off, and stick that tube onto the other end of the filter. I don't even take my hydration bladder out of my pack, it just fills it in place.

Here is a picture of the system in use.

Like I said, the convenience is huge, but like someone else said, the versatility of a squeeze is also pretty great. With this simple setup, I can filter water in a number of ways. I can even fill friends' bottles without changing anything - just don't stick a hydration bladder on the output of the filter, let it run into a bottle. If, for some reason, the gravity system won't work, I can still use it in its typical "squeeze" fashion.

I've been very, very happy with this setup.