They are OK, but there are many ways they can go bad and you wouldnt know until it was too late.
I prefer a Sawyer Mini, or even a sawyer squeeze. No batteries to worry about, can be used in line with hydration packs, good for 100,000 and 1 million gallons, respectively, can be used as a gravity feed filter at a camp for high volume stuff, and they weigh 1.5-3oz.
Ive used and tested nearly every backpacking filter out there (been backpacking for 20+ years), and the Sawyer stuff always comes out on top.
The sawyer bags are great, too. Makes it quick, easy, and they weigh next to nothing.