A/I is generally considered very competitive mostly because of the few numbers of spots, you're often competing with peds applicants as well as other IM applicants for the same fellowship spots, and because it's a nice lifestyle. Most of the A/I fellows I've met have gotten into their fellowship in kind of a roundabout way, often by having close connections with the department (like having done research with them, or doing a related fellowship at the same hospital like pulmonary or rheum, etc).
The training is usually one year of clinical, and 1-2 years of research. The clinical training is mostly outpatient clinics, but the fellows will also do inpatient consults during their clinical time -- they may get called for things such as weird allergic reactions, drug desensitizations, hypereosinophilia evaluation, or ABPA.