Love this name-- I've known one Clif in real life and found it such a fun name when first exposed to it. It's vintage, charming, familiar yet unexpected, and masculine yet soft enough to be wearable. Now if only my in-laws didn't use this name for their dog...
I think Clifford is a sweet and charming name that ages well for a young boy into a man. I love the name Cliff. However, working in education I am familiar with Clifford the Big Red Dog. I love those books and the cartoon. It wouldn't keep me from using the name, at least it's a positive association.
It sounds a bit pretentious and snobby in its full form, but the nickname Cliff has a youthful, fun sound to it without being utterly immature. The name is admittedly a bit dated, though.
Now if only my in-laws didn't use this name for their dog...