As others have already pointed out, there is a ton of information, stats, opinions, designs, reviews etc. on the web with regard to dust collection. At some point in time I think that all of us as woodworkers ponder what we can do to improve DC in our shops, either for health reasons, perceived health reasons, or just to keep the shop clean.
I started off with a shop vac (mostly used to suck up the piles of sawdust after I'd been ww), then upgraded to a 1.5hp DC with a 1 micron canister, again, mostly to keep the piles of sawdust from accumulating in the shop. It did do a good job at collecting sawdust, but as I increased the amount of woodworking I was doing I started to develop a chronic cough. This led me to start exploring my options for improving the DC in my shop. I read all the articles in ww magazines that I could find, as well as the various posts on LJ's about DC. I came across Bill Pentz's site and read it through and through. More or less, several things seemed in common on all the sites;
1) CFM ratings are useless unless you compare them to static pressure
2) The advantage of cyclone DC's is that you don't get dust clogging the filters (with the resulting loss of airflow)
3) There is a big difference between fine dust collection and visible sawdust collection. One will keep your shop looking cleaner, one will improve the air quality in the shop.
4) Adequate fine dust collection requires minimum CFM airflow at each machine (this is where the CFM vs. static pressure curves comes in handy).
I went with a Clearvue cyclone with a 5HP motor; the airflow curve was the best out of all the different manufactures I looked at, however, Penn State Industries was comparable. I no longer have my chronic cough, and I don't have to work through a cloud of dust, even with my mitre saw, which was a terrible dust producer previously.
As you've seen already from the responses to your topic, you'll get a lot of different opinions…my 2 cents is to advise you to read as much as you can on the ww sites, LJ's, and Bill Pentz's site. Then think about what you want a DC for; fine dust or to keep your shop clean?
