I've had a Platypus Gravity Works for several years now. The seal on the dirty bag isn't good. Others have better options. The dry bag style closure Lifestraw uses on their system works much better, but that system is heavier overall. I bought a similar bag off Amazon for my Gravity Works, but haven't had the chance to use it yet.
I've also had the need to backflush a lot on trips with mine. Sometimes there's something you can't see in the water that really wreaks havoc on the filter, and the only way to get it back in service was to pressure backflush with a pump filter. Bringing a syringe for backflshing is a good idea.
The Gravity Works filter by itself does nothing for taste, which may matter or not depending on your water source. I have the carbon filter for mine, but it's really slow. It does work well for what it's supposed to do.
If I had to buy mine again, I don't know that I would, but I don't think there's another clear winner out there. I'll probably slowly replace peices I don't like, and eventually end up with a hybrid system.
For what it's worth, I do also have a pump filter I really like (First Need), but a gravity filter certainly has its advantages, and is much lighter.