Yes, that is what an adrenaline junkie is. It is not impolite to refer to someone as an adrenaline junkie, but it is really only used when someone truly is that way. In my mind, it's not disrespectful, but I suppose it could be made to seem disparaging. However, if you just watched someone bungy-jump off a 300-foot high cliff, you would not be out of line to call that person an adrenaline junkie. It's not common except for people who are like that.

First there is a question of “junky” or “junkie”. Like evuljeanne has written, I think “junkie” would be the more common spelling. “Junky” has an additional meaning as a negative adjective. In general “junkie” is a negative slang term referring to a heroine addict. However I’d say that “adrenaline junkie” is pretty neutral. It could be either positive or negative depending on the speaker’s point of view. Personally I would never use “junkie” but I would use “adrenaline junkie”.