Ryan is correct that filters aren't 100% effective but any reduction in the bad stuff reduces the chance that someone gets sick. So even a 70% effective filter is better than nothing. Another thing to note is that it is pretty easy to contaminate the clean side of the filter with dirty water so you need to be careful in how you use and store the filter.

Ryan is also correct that being careful in you selection of drinking water source is important and, if done well, mitigates the need to treat water. However, depending on where you are, you may not have the luxury of choosing from pristine water sources. In particular, when you are responsible for a bunch of kids, it makes sense to take every procaution. Ryan is welcome to take the risk of drinking untreated water, but it would be irresponsible to give untreated water to your scout group.

I use the Katadyn Hiker Filter (about $60) and have been pretty happy with it. I wouldn't want to pump it enough for a whole scout group but as Ryan pointed out, you can make them do the labor. Supervise them to make sure they don't get the output hose contaminated. There are other filters intended for larger quantities of water which you might also consider if cost isn't an issue.

When I was in the Peace Corps, we had water filters that got pretty nasty over time and had to be cleaned frequently. I was always told that filtration is a matter of taste and doesn't really remove any pathogens. You have to boil or use chemicals to do that.

Drinking from the source is much more satisfying . . . but then, I've had giardia and amoebas, too. You drinks your water, you takes your chances.

You're right- the way I phrased my reply sounds as if I think the filter removes all possibility of infection. But I'm very aware that no filter (that I know of) can do such a thorough job. Still, I am much more comfortable drinking water that's been treated, and filtering is the most tasty and convenient way we've found to do that.

As far as choosing your water source goes, that's very true- it's important to do that with or without a filter.