You need to figure out what you want to filter and what your budget is. The best are reveres osmosis filtration systems and are expensive.
The blue Camco in-line filters are 20 micron and won't filter out any nasties. They're for removing some odd taste at best.
Our Walmart recently stocked a filter that works like the Camco but is a 5 micron filter. That's good, but if you're worried about nasties then you really need something like a 0.5 micron filter.
So it depends on the quality of your water source and what you're trying to remove/
