I've never had a complaint with it, and originally bought it because after doing a lot of looking around, it was the most robust one I had found. Another thing I like is the fact that I can keep it going in the field. The MSR (at least when I got mine) was one of the few that had a cleanable (in the field) filter, and parts kits available. When I got mine (1998), I got an extra filter (ceramic) and two spare parts kits, and haven't needed to buy any replacements yet. As far as my experiences with other filters, I've had various filter bottles that I've carried for daily use of tap/fountain water, and while most any of them are fine for supposedly "safe" water, none of them would begin to stand up to my MSR.

Troy

It depends on the level of protection you're seeking; following filtration with certain chemical treatments gets more "bad stuff" than filtration alone.

It's important to understand the differences in effectiveness between chemical-only, filter-only, purifier-only, or some combination of mechanical-and-chemical; and now, as Brangdon mentioned, ultraviolet purifiers like the AquaStar (or the SteriPEN).

Once you're familiar with the results each process will yield, then you have to decide what level of protection you're comfortable with. Want to avoid exposure to waterborne viruses? Then filter-only treatment is not for you. If usage will mostly be in remote backcountry, maybe you'd be comfortable with filter-only. Conversely, if expected usage will be nearer urban areas, I'd personally want my drinking water purified, and not merely filtered.

Again, for a thorough discussion of water treatment processes/equipment, I recommend the REI article.

The ceramic filter (same medium as the Berkey) is part of the reason that I like my MSR so much, and it's scrubbable too...though Katadyne is a close second IMO.

Troy

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.

I have an MSR Sweetwater, which replaced an old sweetwater. Great system, great value. Used at least once a year for a week long trip, never had any drama. However, I do recomend chemical treatment along with filtration.

I have a Katadyn Pocket Filter to get rid of bacteria and protozoa and to compliment it I have a Katadyn activated carbon pre filter that fits in the outlet hose of the KP to get rid of a lot of the chemicals and bad tastes. I really like the combination and it has worked extremely well thus far and surpassed my expectations.

The Katadyn Pocket may seem exorbitant, especially where replacement ceramic filters are concerned, but its lifespan is extremely long in comparison to a lot of other filters.

You can get the Katadyn Pocket for $159.99 from Amazon (Free Shipping). Right now Amazon also has a deal on replacement ceramic cartridges as well ($79.99).

I am also thinking about buying a SteriPen. Does anyone have any experience with these?

It looks like this would be better used with chemical treatment, but it is compact and if memory serves the retail on it is only about $3 US. I haven't read the specs on filters on purifiers enough to know how it compares, but I thought I'd throw this out as an option. It's called Lifestraw

Note: I have no association with the folks that make or sell this product...just saw in Time magazine a few weeks back.

have you already read this stuff: http://www.equipped.org/watrfood.htm#WaterFilters ? Bit outdated but still very good info.

Is your concern for biological and/or chemical contaminants?

There are many products out there, which address one or both concerns. Costs, ease of use/maintenance, portability are just a few of the variables, one should consider when selecting a water filter/purifier.

If you examine your anticipated needs/parameters, perhaps we could focus in on units that will fulfill your needs.

Pete

Check out these threads:

Steri Pen 1
Steri Pen 2
Steri Pen 3

Pete