After spending a month in the radiology department, it's is definitely in my top-five.What bothers me is that you have almost no patient-contact and (more so) that you do almost nothing curative (well, there's interventional radiology, but you'll end up doing almost exclusively interventional or diagnostic). But apart from that it was a great experience.

My wife was doing a radiology residency at a prestigious program here in the US and after a year withdrew from the program. She loved the program, the people in it and the quality of the learning but she really, really missed patient interaction. My advice here (again unsolicited) is that if patient interaction is at all important to you, don't go this route. She tried to console herself with the thought of interventional too, but it wasn't enough.

She is now applying to Derm residencies and just finished her last interview last week. Imagine going through that process TWICE?! It's been pretty crazy.

But to answer your question, we made a list of the specialties that interested her. We knew we wanted a family so things like Surgery etc were out, although she seems to have an aptitude for surgery. Then we made a pros/cons list for each specialty. In the end Radiology won because we thought the science/technology aspects of it would be intellectually stimulating enough to make up for the lack of patient care. We were wrong.

We chose Derm because my wife loves the pathology involved. She likes being able to diagnose complex internal problems from the clues left on the skin (I'm not a doctor, I'm sure there's a more elegant way of putting this). She is doing a research fellowship at Duke right now and the work she's doing is very interesting. As she puts it, by the time the patient has arrived there they've seen a number of physicians that can't diagnose. She enjoys it quite a bit. It was a good change and I'm glad we made it now rather than 10 years from now.

Anyways, good luck! Immunology sounds like a really fascinating field.