Yes, you can say 'comfy' instead of 'comfortable' in any informal situation. I have a feeling that this is a particularly British thing, though. I recently read a blog by an American woman talking about all the things that really irritated her about the way the British speak. Top of her list was the way we abbreviate words like biccy for biscuit, pressie for present, cardi for cardigan and so on.

As an American, I can weigh in on this question. You can use comfy when talking about objects, especially a couch, but it would be awkward when talking about a person. Ex: It would be awkward to hear "I feel comfy around him," in that case I would say comfortable.

I agree with Lynne. "Comfy" is very common in the U.S. but it is restricted to a) the feeling of physical comfort, and b) the physical things in which you are comfy, such as furniture, beds, pillows, cushions, and warm clothing.. It is informal, and it carries a sense of homeyness, domesticity, and comfort at the expense of style. It is the same kind of word as "snug" and "cozy." It suggests the idea of wriggling around a bit to find a comfortable position. My (adult) son refers to the recliners in our living room as "the comfy chairs." "I've turned on the gas fireplace and you can just sit here and get comfy, while I go to the kitchen and make some cocoa." "I know it's getting shabby, but I still like my comfy chair." "What's better than making a comfy home and bringing up some cute kids and knowing nice homey people?"--Sinclair Lewis, Main Street "the Cat was sitting quite comfy inside the Cave."--Rudyard Kipling, "Just So Stories" "At last Mother said to Father, "Now, dear, if you're quite rested, and quite comfy..." E. Nesbit, "The Railway Children"