During my AT thru-hike I used the Katadyn as my sole water filter from Springer Mtn thru the end of the Smokies at Hot Springs. It served its purpose at the beginning but I noticed 2 big drawbacks that led to me replacing it.
1. It was easy to pump and had a good flow rate at first. Unless you are filtering from a fairly deep water source, however, the suction ball will sit on the bottom of the water source and suck in any dirt/debris. The filter does its job and keeps that from ending up in your bottle, but all that dirt/debris builds up. As time goes on it gets extremely hard to pump the handle and is noticeably slower and more of a workout to get your water. I tried cleaning the filter in sinks when I got the chance but never noticed any improvements. At Hot Springs I went to buy a replacement filter and was shocked to see they're $40ish each. I'm sure you could get them slightly cheaper online, but you don't always have that luxury on the trail. I ended up buying a $40 Sawyer filter starter kit that gave me a filter, cleaning syringe, and 2 bags. That one purchase, with regular, free, and effective cleanings (backwashes) lasted me the rest of the AT.
2.. My second complaint about the Katadyn is that you need pooled water to filter. A lot of the sources on the AT, especially as you go north, are running water sources with little to no pooling. The Sawyer, in conjunction with a jerry-rigged water bottle scoop works perfectly in shallow flowing water.
3. (I know I said 2 complaints...I probably wouldn't have replaced it if it was only this complaint) It's relatively heavy and bulky.
So the Katadyn works. And for casual or short trips, it probably works just fine. But if you're counting ounces, cubic inches, dollars (over the long run), or expect to use shallow flowing water sources, it's just not something I'd recommend.