Regarding HEPA filtration, and its importance on a vacuum, I think Mike has given a good answer, but I will add to it. The settling time of a particle in the air is inversely proportional to the size of the particle. Larger particles settle faster, while smaller particles remain airborne longer. The problem comes when you run a vacuum. It attempts to pick up all the dust from the carpet, and catches what it can in the bag or filter system, and the rest passes out the exhaust and becomes airborne. If you run a vacuum with no bag, you know what happens - after vacuuming the carpet is cleaner, but the air is filled with dust. Since most of this visible dust is large, it settles out fairly quickly, though.  
 
If you vacuum with a bag that can catch down to 30 microns  (a standard bag) the same thing happens - the carpet is cleaner, but the air is dirtier in terms of small particles, though the particles will be too small to see. Because they are smaller, they will take longer to settle. Use a vacuum with a filterair bag, which filters down to 6 microns, and afterwards the air will be filled with particles smaller than 6 microns. These take a very long time to settle, up to 4-12 hours, but are invisible, and unless you are allergic to them, cause no problems.
 
Next, lets look at specific allergies. If you are allergic to pollen, most pollen is in the 8-10 micron range, if I recall. It won't be caught by a standard bag, but it will be caught by an "allergy bag". Some people are allergic to dust mite feces. These particles are .3 microns or so. No bag will catch them, and they will be caught only by a HEPA filter. Worse, these particles take up to 12 hours to settle.  For a person with a severe dust mite allergy, if a home is vacuumed with a non-HEPA vacuum, that home will be uninhabitable for about 12-20 hours.
 
I used to have to vacuum and then open all the windows and leave, or have my wife vacuum when I was not at home. Now that I have a HEPA vacuum (Air-way), this is no longer a problem. As I vacuum I actually feel better, not worse. Wow was I surprised the first time that happened! Furthermore I am no longer afraid to vacuum things like the pillows or mattress. Now I vacuum then regularly, which allows me to sleep much better at night.
 
Old timer, you are fortunate not to have allergies. On the other hand, while you may not have dust mite allergies, but there are people who do. For those people a HEPA vacuum is of extreme value, and I for one would never go back to a regular vacuum. For someone in the business, it would behoove you to learn a little about this. That way, when a customer came in, you could ask them if they have allergies, and you could determine if they have an adverse reaction to vacuuming that could be remedied by a HEPA-rated vacuum.  Once they realize the improvement it will make in their life, they will appreciate it a great deal, and will be willing to pay the cost difference to get a HEPA rated vacuum.
 
So, in summary, a person with no allergies has no particlar need for a HEPA vacuum, or even Filtrette bags. A person with pollen allergies should be fine with improved bags. A person with dust mite allergies should use a HEPA rated vacuum.

Quote from Dyson_Chris on 10/24/06 at 5:19pm:

 
That's actually a cutaway of the Root 12 Cyclone unit from the DC08 canister.
 

My Dyson rep brought in a sample of this machine today. I love the colors, and the hose release system is greatly improved.
 
However the bristles are EXTREMELY stiff. I have gray velvet plush in the store, and after a few passes with this machine, there was a huge ball
of carpet fiber in the tank. It actually fuzzed the top of my carpet because the brushroll is so aggressive. It'll definitely clean well, though. Personally I would rather have a machine that's more gentle on the carpet, but can still do a good job of cleaning.

Quote from Dyson_Chris on 10/24/06 at 7:30pm:

 
LOL! Dyson: dammed if they do, dammed if they don't!