One would have to compare the performance of a Rainbow to another canister vacuum, not an upright. Kirby isn't convenient to use with attachments, and you certainly can't wash your kitchen floor with a Riccar. The Rainbow will shampoo the carpet (and not just rub bubbles into it like 'dry foam' methods). The Rainbow will unclog drains and pick up spills. The Rainbow has a motorized mini power brush for stairs and pet hair. The Rainbow's high quality attachments are easy and dare I say, fun to use. Sure, taking fifteen seconds to fill the water pan before use might be a chore to some. Emptying the basin may take a moment (I've always emptied it into the commode and never had a clog, ever) but all the dirt is out of your house - not in your closet. Best of all, the Rainbow doesn't stink like last month's dirt - and you have no expense for consumables. Rainbow's HEPA neutralizer is washable.
My Rainbow has been running on air wash mode since October, in my bedroom, continuously. I use distilled water in it and have to refill it daily as it adds moisture and humidity to the dry heated air in my home. When I want to vacuum, I attach the hose, switch to high speed, and clean. The fifteen foot electric hose means I never move the cleaner once it's in a room. The hose is long enough to allow me to clean everything on as well as above the floor.
They are certainly not for everyone, and I do have several 'bagged' type cleaners as well as the wonderful Hoover Air Cordless for quick cleaning jobs.