My parents had one for over 20 years. It always worked great, and I didn't think much about it.
When I was in college, I lost my job and started cleaning houses while looking for a regular job. I was diagnosed with asthma and had a really rough winter. The problem is I was using the vacuum cleaners provided at the houses. I never knew it, but that water tank vacuum cleaner was keeping me from asthma problems.
Changing a bag or getting a clogged hose would set off asthma attacks for a week. But with the rainbow, all I had to do was empty a water tank, and I'm not allergic to mud.
I currently have a Eureka vacuum with a canister. It's better than the bags since I can see when it is full and carefully empty it without too much dust in the air. I still have more allergy problems than I used to.
I would love to get another Rainbow vac, but I can't afford to buy a spendy one, and even the used ones are usually over $500. I almost had a good trade for one on craigslist, but after setting up the trade, the person stopped responding.
I can see why a lot of people wouldn't want one, but if you have allergies and have trouble with emptying out the dirt, a rainbow might be a great choice for you. Also, since the dirt is going through the water bowl, you don't need to buy and change filters a lot.