Market for rheum is good. Large markets are obviously more saturated, but there are a ton of jobs out there. Just go to mdsearch or any other large recruiting website and look at the number of job openings. While this is obviously NOT indicative of the actual number of jobs present in the market, it is a good barometer to the overall health of the job market for a particular field. Rheum, if you take a look, has a great job to fellows ratio. But, it may be more difficult to find a single specialty practice, since those are far more desirable for a variety of reasons.
From what I've heard, rheum is getting more competitive (though still not very competitive overall), and the pay is going up. For me, the thing about rheum is that tons of treatments are looking to pop up in the next 5-10 years. It may be a long shot, but if they ever come up with some biologic that you can use to treat osteoarthritis, then it'll be the next lucrative IM sub-specialty.
Also, you treat overall pretty healthy ppl. You do have your crazy fibro patients, but to be honest, once you develop a relationship with them, they are some of the easier patients to deal with. They see you once every 4-6 months and you tell them to keep doing what they're doing and you sympathize with them. One of our attendings runs a solely FM practice, and it's quite possibly the easiest clinic I've ever had as a resident. The interaction is literally like this. "Hi Ms. Smith, how are you doing?" "Terrible. I'm hurting all over." "Ok, well keep exercising and taking gabapentin. We'll see you in 6 months." "Ok, sounds good."
And lastly, I find it fascinating. Lupus nephritis? Scleroderma renal crisis? Familial Mediterranean Fever? Love it.