Ward off Giardia with any of these devices. MSR gravity filter is painless except for having to back flush it every 8 qts. or so. Great for group use. Katadyn filter is pretty reliable except you might need to prime it by filling inside with little bit of water the second year of use. Steripen wins hands down in the weight contest - I like it a lot. On longer trips in groups we carry 2 types of water treatment in case of malfunction.