I have an old Katadyn Pocket which has been great, and 3 weeks ago I got a Katadyn Mini.
The Pocket is quite a few years old and it has never had a replacement cartridge and is still going strong. Katadyn put a very abrasive cleaner with it, (I think they want to sell more cartridges) but it has never been used. A higher grit bit of sand paper is much better and smaller to pack. The ceramic cartridge is impregnated with silver to aid with killing any nasties and it think its down to .2 micron filtering. (going from memory so I may be wrong)
I wanted a smaller filter to pack, and I found a local who was selling the Mini, so I bought that from him. It uses the same type of ceramic filter as the Pocket only smaller. It also takes more strokes to get the same amount of water, but it is half the size of the Pocket. Unfortunatly the previous owner use the cleaner that came with it, but it still has plenty of life left.
I highly recomend either of these. They also do a filter called a Combi which is midway between these 2 in cost and size. It has a carbon filter cartridge as well as the ceramic cartridge. I didnt see the need for this though.
The Katadyn Pocket was the best filter I could find at the time, and I havent found any I would replace it with apart from the Mini. The Katadyn Mini is the filter I should have bought the first time, because its just as good as the pocket but smaller and cheaper.
If you are filtering for 1to 3 people, the Mini is great. For more than 3 look for the Pocket.