It's replacement filters are expensive and only last for about 20 gallons or so. It's only one stage of filtration, so not good enough for saltwater aquariums. And with how often you have to change the filter, you'd be saving money by just buying an RODI unit (the filters of an RODI unit only need to be changed about once a year).
You can get a good RODI unit (5 stages or so) for around $150.