Vacuums can be most easily defined as being an AIR pump and in the process of shaking the floor and pumping air through the system, dirt & debris are carried along as a by-product. That by-product(dirt) is then filtered by a vacuum bag which may or may not capture all that has been removed from the carpet.
The best vacuums for Carpeting will always be the ones that pump air the fastest (have a high CFM rating) and contain the most particles in the bag. The faster the vacuum pumps air the less time it will take you to clean, which to me always = better!
The problem with most vacuums is that they are not only lousy air pumps they are even worse at filtering what they have removed. Although you will always hear claims of HEPA filtration, nobody really tests the performance of said products over time. Almost all vacuums have a high grade filter of some sort on them, but having it is one thing, FORCING ALL OF THE AIR & DIRT through it is another. Imagine trying to push air at 100 miles an hour from a small tube into a dirty filter. When the air can't get through, it will either spew out into another unsealed area, or just bogg down your airflow causing clogs and other problems with performance.
In my opinion The best vacuums do three simple things.
1. Clean carpet at a reasonable rate
2. Keep what they suck up in the bag
3. Dont break down every other time you use them.
This is what amazes me about buyers magazines, a lot of the time the number two or three vacuum will out clean number one, but won't be rated as high?
If it doesn't perform its primary function,(cleaning carpet), what good is it? It would be like having a lawn mower that cuts half the grass when you make a pass but sure is cheap & quiet so it's a best buy.
Reliability has got to play a role, sure you can get a top rated Eureka for $150.00. But price is one thing and cost is another. The initial investment (price) is good but the cost is horrible as they will not be reliable or perform to a good standard. How do you think manufactures arrive at that price point? By cutting corners on items like the bearings, brushroll, grade of plastic, etc. Vacuums such as Kenmore, in my opinion shouldn't even be rated, most of them are downgraded Panasonic vacuums with a Kenmore name on them. That's beside the point the real reason you should'nt buy a Kenmore is PARTS & SERVICE. You will have a problem and it will cost you two to three times as much, why you ask well Sears doesn't make parts available to anyone but themselves and they charge $74.95 flat rate labor(at least in Tulsa that is). Even for a belt change, Trust me on this one go to your local vac shop talk to someone who's been there at least five years or so and you will thank me for it.
"The bitterness of poor quality remains long after the sweetness of low price has faded away"
This doesn't mean you have to pay $2,000 to get a good vacuum, lots of customers have done that only to find out they are not happy with their product. But don't kid yourself into thinking that a testing magazines "best buy" vacuum for $150 is even close to something you could buy in the $300 bracket.
Back to function. What makes a vacuum powerful? you ask, while amperage does play a role it's the SIZE OF THE IMPELLAR(fan)that is most important. Let me explain, if we take a motor that draws 12.0 amps and has a 10 inch fan it would produce much higher power than a motor that is 12 amps with a 2 inch fan. The bigger the fan the better, as long as it is spinning at a high rate of speed.
Examples: Oreck has about a 3 inch fan with a 4.5 amp motor
Kirby has about a 7 inch fan with a 7.5 amp motor
In this instance a Kirby will produce almost 70% higher air volume or CFM. A lot of vacuums have 12 amp motors but they either have a very small fan or they are too far away from the carpet.
Don't be fooled by advertising, use your head, the closer the power is to the carpet the better it will be. Keep in mind most of my writing pertains to cleaning carpet as any $99.00 canister can clean a hard surface, you can't move air through wood or concrete so CFM is not as important on hard surfaces.
Another thing that is confusing is the fact that 90% of vacuums today are actually canister in nature(meaning that all the air and dirt go through the attachment hose all the time). A good rule of thumb is if it has a hose on board that you cannot remove you have a canister disgused as an upright. While these types are convenient they are not as powerful(on carpet) as a true Upright.