Well, if you are thinking about buying a Rainbow, one important aspect I would be thinking of is how much will you actually use it? Yes, the Rainbow & other water filtration vacuums do a great job of cleaning, & ultimately the water filtration is the best way to dispose of dirt if you want to use a bagless vacuum. The problem with them lies in the fact that it requires extra effort to set up & clean afterwards. It's not just as simple as press a button & the dirt falls out, & you clean a filter once a month, like a Dyson requires. And they are not as convenient to take out & clean up quickly when company calls & says they will be coming to your home soon & you have a messy house that needs cleaning.
For that reason, make sure you keep a spare vacuum that you can use for quick cleaning jobs & for maintenance cleaning. And definitely DO NOT trade in your Miele to lower the price of the Rainbow! Chances are you can sell your Miele locally or on Ebay & get substantially more money than the credit they will take off the vacuum's selling price. And chances are, the dealer will recoup their money they took off the vacuum by selling yours anyways, so keep that money in YOUR pocket, not theirs.
Make sure when the dealer is in your home & showing you how the vacuum & optional attachments & uses it has, think carefully about whether you will REALLY use those attachments. For instance, how many people would actually use the carpet extractor attachment & extraction clean their carpet at home? And make sure the dealer knows you do have other options, & they aren't the only company that makes & sells a water filtration vacuum. There are other competing brands, like Sirena & Quantum, that make a water filtration vacuum that cleans just as well. As Mark said above, keep turning them down until you make it clear to them that either they have the lowest price or no sale!
If you are thinking of buying another brand of water filtration vacuum to save some money & you don't need all the extra functionality a Rainbow offers, here's some tips:
1. Make sure any water filtration vacuum you buy has a HEPA Filter on the exhaust, & don't buy if it doesn't have that feature. The water filtration vacuums may be the cleanest way to own a bagless vacuum, but unfortunately their filtration claims aren't exactly true. Any dirt that cannot be wetted by the action of the water....flour, baking soda, carpet deodorizers, fireplace ashes, any fine powder dirt, etc....will bounce off the water, go right thru the separator, into the motor & be exhausted back into the room air you are cleaning. That's the whole point of the HEPA Filter on the exhaust, to trap the fine particulates the water filtration cannot trap.
2. Make sure the brand you buy has parts support & availability from a local vacuum shop. This can be a HUGE issue, especially with water filtration vacuums, as there are many unknown or unheard of brands that are so new that the shop's distributor they buy their parts from doesn't stock parts for that brand. Also beware that many vac shops won't repair Rainbow or other water filtration vacuums, because they don't believe that such a vacuum is a good idea or design, they believe it's dangerous & they refuse to repair it. So make sure you have a local shop that is also willing to cooperate & work on them. I'm sure the last thing you want to have happen is to buy a vacuum & then every time it breaks spend hours in a car taking it to be repaired & then to pick it up again.
3. Consider buying a extended warranty if it's reasonably priced. You are making a substantial investment in cleaning equipment for your home, & since there's moisture, a motor & likely computer boards involved, it's not unheard of to have a vacuum break down unexpectedly, even if it's maintained well. Just make sure you know the EXACT terms & conditions of the warranty & what it covers, & more importantly that there's a way of getting the warranty honored if something does break & that it's convenient to use the warranty.
Good luck with your purchase & enjoy it.
Rob