I went to the sights Dr Enright suggested as well as others and was not able to find any mention of the Rainbow vacuum. While I did confirm some expensive machines did no better than less expensive ones their studies failed to include the Rainbow or any like it. If you can find any study that includes the Rainbow, I would sure like to know.
The Rainbow uses a water based filter instead of dry porous bags/filters and claims to be substantially different from other vacuums in both performance as well as an air cleaner.
Given the lack of any known study, I went to
www.epinions.com/reviews/hmgd-Large_Appliances-All-Rainbow_e_Series, as well as
www.vacuumwizard.com/rainbow-reviews.html. Out of hundreds of reviews of people with allergies and similar ailments, the vast majority loved the system and said it was well worth the price. A number of people gave testimonials about the Rainbow substantially helping them. Quite a few stated they were able to reduce and/or eliminate medications after the use of this product.
Dr Enright's stated "In fact, while vacuuming carpets, all types of vacuum cleaners use a rotating beater brush which temporarily increases airborne allergen levels. Therefore, people with allergies should not perform vacuuming and should stay out of carpeted rooms where vacuuming has been done during the previous hour or two." I do not know if the Dr is aware the Rainbow was the ONLY vacuum that had the capability to be used for "clean rooms" which are 10,000 times cleaner than medical operating rooms. Rainbows not only clean 99.997% of the pollutants/bacteria/dust mites/dust particles in the air, they also trap them so they don't come out the exhaust.
If Dr Enright had stated "I know nothing about the Rainbow but this is what I think about vacuums in general", I think his comments are accurate. However he does not appear to know the strengths and weaknesses of the Rainbow vacuum in particular.
I hope Dr Enright or others can come up with studies or facts that could enlighten us all or provide some info that is a bit more relevant to the question.
Is the Rainbow worth it ? A majority of people who have tried it say "YES !".