Since I'm selling these things as a side job (well more a the fact that I owned two and they wanted me to do it and it fit my schedule). I am not going to lie to anyone or pitch them one bit.
I have played around with them quite a bit if the airflow is poor, it seems way off, but a replacement filter seems like a good plan of action if the seperator is clean. Sounds like it has less airflow than my one speed E Series?
A trick for non wettables? Add a drop of dish soap to the bin. Remember the seperator will keep the water out of the motor, that is what it was designed for. It imakes a foamy mess but works. You can drop these things sideways and as long as they're running water is not getting into that motor. The foam from the soap traps the non wettables.
I still wouldn't recommend it for that purpose.
As far as storing on water basin? That solely depends on how well you dry the water basin. It is their precaution, but if you dump and thoroughly try the basin your fine.
Take it from my perspective, I have maybe 20-30 Fantoms, 2 Dysons, 4 Kirbys in the garage, 2 Rainbows, and various other machines. The Rainbow will do a fantastic job and last forever if you care for it. My single speed E series HEPA filter is still perfectly white, and that is after vacuuming disgusting black crap my Dyson DC33 left behind. Dyson is not perfect in this respect because the internal filters still need cleaning (which leads to loss of suction regardless of their claims), and I don't feel their brushrollers are agressive enough..
Is it the most convenient? No. However after the dust cloud of emptying the Dyson you begin to understand what they do what they do and why the machine is still around after all these years. The more you use it the less inconvenient it seems. Their current powerhead is very nice for a cansiter.
Is it expensive? No doubt.
If you have the time to use it properly it will perform as advertised. If you maintain it it will last a long time. To any buyer, be it on ebay, craigslist or a dealer, it you take care of it it isn't a 6 month machine like your $50 disposable, but you have to decide if you like it because it probably will last 20+ years.
The other positive is it holds value well. Rainbow gives you a good trade in value. They sell at commendable prices online (unless someone uninformed unloads it cheap). It is a fairly safe buy in that respect.