I've read very bad reviews about the First Needs Deluxe...people are either super enthousiastic because it works perfectly or really desappointed because it broke down very early. It's a pitty though, because it is a great design and price...:-(
I've only read rave reviews about the Katadyn Pocket though. Plus it's their 75th birthday so you can get it in a neet black case with a swiss cross and "75th anniversary pocket" written on (looks nicer than the blue one). More seriously, it's a bit heavy, and expensive, but it will filter 50'000 liters before having to change the cartridge, it comes with a pre-filter/floater (like most) and you can add a carbon post-filter to eliminate bad tastes, smells and chemicals from muddy or polluted water, it's got good flow rate, it's very resistant (as long as you don't drop the filter part without it's case off a cliff...in a review I read the reviewer accidently did that when cleaning it), it's been around for 50 years (always a good sign) and comes with a lifetime warrenty (but 20 years on the filter I think). You'll just have to treat the water with chemicals like micropur (to stay swiss ;-)) or aqua mira after having pumped if you're in a virus contaminated zone. The ex-PUR (now bought back by Katadyn) line is worth a look as well, they're cheaper and lighter, and also have a good reputation. I think I read problems about clogging and not being able to clean the filter though (but it was maybe for another one). But unlike the Pocket, you might have to change the filter once in your life ^_^
The pre-mac ones are cheap, but can't filter as much water before changing the filter. They've also got a awfully slow flow rate (5 minutes of pumping for 1l of water for the little model...that would be 15 minutes of pumping to fill up my platypus!), so they wouldn't really be suitable for a group. They do purify the water though, that means you don't need to carry extra chemicals or wait for them to have their effect (about 20-30min usually). And they also fit in a pocket, unlike the katadyn pocket! I think I'd get one of those if I needed a filter for just one week in Africa or somewhere like that then not use it anymore.
Do they have any carbon in them or anything that will eliminate bad taste and smell?
So from the information I gathered up:
For a group, where weight and bulk are not so much of a concern, but you need alot of water, and in a non-contaminated zone (virus contamination I mean) : Katadyn Pocket or something similar (MSR miniworks, PUR guide/hiker...)
For personnal use in a very contaminated zone, where weight and bulk are important (ultralight backpacking for instance), but you don't mind pumping a bit longer, one of the Pre-Mac range of filters could be the one for you.
Again, I think the First Needs deluxe would be the perfect filter for anywhere : compact, light, high flow rate, fits on most bottles, purifies, cheap...But then I wouldn't want it to break when I need it most!
[I just want to precise : I've only seen some of these in a shop, never used them. What I wrote here comes from the knowledge I picked up reading around]