I usually fill my Sirena to the top of the post or just below the max line and I have had no problems. I do think there is more surface to clean underneath due to the shape of the basin, but all in all it is a decent machine.
I will agree with Mark that Rainbow has had longer to refine their machine, however, the Sirena doesn't cost as much as a Rainbow.
In the mix you will not get direct connect wands. You will have a separate non-electric hose and an electric hose. The attachments, though of decent quality are a pain with that pin that has to be pushed to release, except for the dusting brush which is always falling off because it lacks a hole for the pin.
It does not, at least at this time, have a shampoo attachment, but is rated for wet pick up. I have mentioned several times that I am not in love with the power nozzle on the Sirena, but I think it is more my preferences than the actual design of the nozzle itself.
Wish list:
Longer power cord.
Headlight on power nozzle
More and clearly marked height settings on the power nozzle.
In the end, I like the Sirena. I did get a good price and would buy one again knowing what I know now.