If you feel like sending such a note — then definitely send it! There's absolutely nothing inappropriate about doing that.
So I agree with all the answers above, which made a number of great points. But let me add my five cents anyway. As a college teacher myself, I've received a number of such emails (and even postcards) from my students — and I enjoyed every one of them, and each one made my day! Each such message made me feel happy and appreciated, and as humans we all need that.
Look, ours is not an easy job. It has its perks and its moments, as well as a fair share of its challenges. Whatever we do, we can't predict the outcomes of our work and our efforts — because we're working with people, our students, not with inanimate objects. So feedback is extremely important to us. Maybe it's just me, but I feel that we get more than enough negative feedback, sometimes deserved, sometimes not. We certainly need your positive feedback too!
That's not the same thing. Your feedback, because of being unsolicited, but rather something that you felt compelled to do as a sign of appreciation, is much more valuable than the officially requested student evaluations. Besides, the tone in evaluations is typically different — students tell the school about the instructor, as opposed to your writing directly to the instructor. Not to mention that evaluations are anonymous (which can be a good thing and a bad thing), and that your comment there is just one in a pile of all the other comments, which often paint a mixed picture.
One more thing. In fact, by sending such a message, you will be doing a tangible favor to him — which, judging by what you said, the professor deserved, so nothing's wrong with that. When we're evaluated by the school's administration for the purposes of promotions, contract renewal, tenure, and such, for a teaching faculty his/her teaching portfolio must include students' perception of his/her work. I save all these emails, and I always include them in my portfolio. And I know from informal conversations with administrators that, while not decisive, they do play a tangible positive role.