The backpacker-sized filters like the Katadyn are great for backpacking. I don't even carry water with me when I go backpacking - simply stop by a stream, puddle, whatever and I'm good. The rep told me that since it has a micro filter that is Iodine treated, I could even put it into raw sewage and it would filter all the bad stuff. Not a good idea, but if you're desperate I suppose it wouldn't matter...
Anyway, a few things to keep in mind - the have iodine-impregnated filters and they must be replaced every so often.
The size of the pores in the filter make for VERY slow work! Keep this in mind if you need more than a quart or so at a time. You'll spend a good 5-10 minutes of WORK to get a gallon. Therefore the larger lever-action filters may be better where volume is more important than packability.
Also the hose isn't too long, so you really need to get down close to your source to filter. I always had to get down on my knees to ensure the hose would reach. I'd consider a longer (food-grade) hose to go along with those extra filters.
That reminds me, I need to get a new filter for mine!