For me it depends on the source. I've used one for over 15 years in the Sierra's and have never been sick, my first one was when they were still under the PUR name. I just got the hiker pro two weeks ago for an earthquake kit specifically because I have faith in them, from a mechanical aspect.
There are smaller filters and more efficient filters out there but this gets the giardia out which is the primary concern where I live and I've never had it fail, so I put a lot of value in it. I have broken one once but that was user error, fell canyoneering and it took out my slr camera at the same time. I've added Iodine and Chlorine (sperate occasions, not at the same time) to kill viruses if I'm pumping from questionable sources, namely stagnant water or obviously contaminated water. A steripen would work well too as another level of protection post-filter. In general I try to only pump from clear, swiftly moving sources.
The gravity fed filters are a very attractive option if you haven't already checked them out.
Also helps to strain the water through cloth or some other medium to remove as much of the particulates as possible before running it through the filter. The more filtered your water is the longer your water filter will last (Duh, captain obvious). Or at the very least try to draw from the surface of the water source so you dont suck up debris.
And like OCD7785 said keep your "dirty" containers away from your "clean" containers and never shall the two meet. Giardia and other bacteria can contaminate your clean containers
Everytime I filter water, I think about my youth and how we used to drink from any stream or puddle and I dont ever remember getting sick. Of course this was just dumb luck and the area I grew up in but damn I miss being to dumb to be scared
I recently purchased a Sawyer Mini and love it!! I just keep a couple empty Coke bottles in the Expedition, and refill every time we come to a creek.
My favorite so far was last trip out, I filled up at a river with lots of sediment flowing by, and found I inadvertantly let a little creek worm/bug thing in the bottle... of course I drank it all, just to show the kids the filter worked.