I just got back from a week of backpacking with a group of 11. We had 4 filters to start but the first day 2 of them crapped out. But that wasn't a problem as 2 filtersfor 11 people did just fine. I had a MSR Dromendary Bag - http://www.elk101.com/store/msr-dromedary-bags - that I would pump into so that we didn't have to pump as often and the filters held up for all 6 days without a hitch.
I'd second what bentelbow said - try to make sure you're filtering stream water that has less silt if at all possible. If you have to filter from lakes, do a pre-filter with a coffee filter or something to remove the sand and other big particle stuff.
My stomach is like a rock! haha I just find a spring and drink straight from it. I've grown up doing it so I think my stomach is used to everything nasty. And I'm sure that I have almost every parasite and disease known to the rocky mountains haha. I've only gotten sick once and it was for about a month then it cleared up. What doesn't kill you makes you stronger.
