I'm procrastinating because I rather be fishing (or writing about fishing) than catching up on some work. Here's a review...
Katadyn Hiker water filter - $54.99 on Amazon.com
I've never met another fly fisher who brings a water filter to the river. I find this surprising, because modern filters are light, easy to use, cheap, and can turn a muddy pool an inch deep into a delicious refreshing drink.
I drink a lot when out an entire day, and carrying that much water around is a hassle. After buying the Katadyn Hiker water filter for backpacking/camping, I decided to add it to the long list of stuff I carry on the river. It's well worth the extra bulk, because as I mentioned, it's significantly lighter than a day's worth of water. And if you're fishing trout streams, you're guaranteed a really cold cup of fresh water... much better than a bottle of warm water that's been sitting in your pack all day.
As far as use, the Katadyn (and most other modern filters) is easy. It comes with a zippered pouch, the main filter, and two hoses. One hose is attached to a weighted pre-filter that sinks to the bottom of whatever water source you're using. The other, the outlet hose, is stored in a clear plastic bag (that comes with the unit), which keeps it from touching unpurified water. Both hoses are connected, and after a few pumps you have filtered water. You have to take care not to get the outlet hose in your unpurified water source. I should mention that I have used this unit on the Yuba, Putah, the Little Truckee, and the Middle Fork of the Feather without ever getting sick. It removes bacteria, viruses, and protazoa, as well as many dangerous chemicals from pesticides to trihalomethanes.
Maintence is fairly minimal. After you get home, run some clean water through it, take it apart and let it dry to prevent mildew. While this is easy to do, it's definitely easy to forget.
If you're a weight-concious angler like I am, I recommend the Katadyn filter. If you don't mind hauling a heavy Camelback or a couple of bottles of water, the 10 or so minutes it takes to filter a liter of water may not be worth it. Overall though, I think more anglers would suffer fewer backaches at the end of a day if they brought one along.
Pros: fast, cheap, light, easy to use, includes bag and accessories. Cons: plastic construction could break eventually, you need to buy replacement filters every so often (they run about $34), and Katadyn hasn't figured out how to have it purify stream water into Sierra Nevada Pale Ale yet.
I have one and it does work good. I use it mostly when out in the back country when hiking. "Katadyn hasn't figured out how to have it purify stream water into Sierra Nevada Pale Ale yet." Bob and I just look for full beer cans in the local waters.