PUR Hiker
Pros:
light weight
easy to handle
(Advanced Backpacking, A Trailside Guide -by Berger
)
Check the size of filter used. Giardia is 4 microns, bacteria are about 2 microns. The smaller the holes in the filter, the more effective it will be, but the trade-off is that small filters get clogged more often. Most people choose 4 micron filters, they're probably good enough.
Check the rate of filtering, ease of cleaning the filter, convenience of use
Look for a pre-filter which will filter out large particles to lessent the chance of clogs.
(Hiking and Backpacking by Eric Seaborg and Ellen Dudley
)