Seconding what PPGfrog said, most are designed to go under the sink. I use a Watts brand and am happy with it. I've got a tee in my line that splits off to sneak around the back of my kitchen cabinets over to my refrigerator so my ice machine also makes RO ice. 3ish gallon tank under the sink takes up a fair bit of room too.
These filters produce water at a trickle so the tank is important, without that you'd be waiting minutes as your cup slowly filled direct off the RO output.
Seconding what PPGfrog said, most are designed to go under the sink. I use a Watts brand and am happy with it. I've got a tee in my line that splits off to sneak around the back of my kitchen cabinets over to my refrigerator so my ice machine also makes RO ice. 3ish gallon tank under the sink takes up a fair bit of room too.
These filters produce water at a trickle so the tank is important, without that you'd be waiting minutes as your cup slowly filled direct off the RO output.