My T has said it to me once, kind of in passing actually. It seemed very natural and spontaneous at the time -- completely appropriate in the context of what was going on. It didn't cause any transference problems or dependency issues. It just was an honest sentiment and completely fine -- kind of like how you say I love you to a family member I guess. Just genuine affection for the person.

I think the appropriateness of it would depend on the kind of relationship that exists. Obviously it would be inappropriate if it was meant in any way other than completely platonic. And if a T was working with a client with transference, attachment, dependency issues, it would probably cause more issues than help. But with a T who has good boundaries and a client who can accept those words at face value without reading more into it than it is, it is probably not a terribly big deal.

As with all things therapy, the context and relationship dynamics are everything.