I have been selling slushies in one theatre for 5 years--I love them, they sell well and if your lucky like I am and deal with a supplier who is in same city as a manufacturer of the slush base and flavors,which keeps shipping costs way down, it is as profitable as soda. I live in an area where winter seems to last 9 months, but sell as much or more as I do in summer. I get the same price for slush as soda--selling 12 and 20 oz, $1.00 & $1.50, but realistically could charge more. I use sweatheart trophy styrophoam cups for slush, keeps down on sweating on humid days, don't freeze patrons hands on cold days. It is a bit harder mopping spills than soda, but that is the only disadvantage I see. It has increased drink sales, not hurt soda sales.
Equipment new can be cosly, but I just purchased a used unit for $450 for another theatre we just started selling slush in. I see the is a company selling them new on Ebay, prices there seem fair for new.