I did a medicine prelim year, but most of my co-residents and friends in subspecialties within medicine did TYs. My disclaimer is that I don't know too many people who moved twice and went out of their way to do the absolute easiest year that they could. Maybe at some of these you're treated like a MSIV - I'm not sure....
From what I've gotten from my colleagues. I did work harder than almost all of them. Overall, the TY does tend to be more relaxed, but a lot of this is because you have more elective time to explore other specialties. On any TY, however, you will have a certain number of core floor blocks and ICU rotations where you are expected to function just like any other intern. In fact, at programs where there are both TYs and categorical medicine interns, I've heard that the TYs sometimes get the less desirable rotations and schedules because programs want to keep up the morale of their 3 year residents. Up until this point you've been a learner paying for your education. As an intern you're getting paid somewhere in the neighborhood of $50k. This averages out to $8-10K per difficult inpatient rotation on the average "cush" TY. You're expected to do a level of work comensurate with this and you have a significant level of responsibility for a lot of patients. When you're called in the middle of the night with a crashing patient, nobody cares that you're a TY. Your senior may be nowhere in sight, or more likely is taking care of another crashing patient. If you go into most - admittedly perhaps not all - TY programs with the mentality that you can act as a 4th year medical student, you will be in for a very, very rude awakening. As I mentioned earlier, most people will get the hang of managing the vast majority of common inpatient clinical problems within the first two or three months. After that, additional floor time may start to feel painful if your intention is to do an outpatient specialty. As for procedures, there are plenty of central lines to go around at most programs. Sure, you're unlikely to ever do another one in your life, but it's fun and rewarding to get these skills as well as a number of others. After you've done a thoracentesis or two (or some of the other less common procedures), it's nice to give these to your categorical colleagues no matter whether you're prelim IM or TY since they will need these for hospital certification and you won't.
Finally, I'd think long and hard about moonlighting in IM/ED during ophtho residency. For starters, your contract forbids this at many places. But much more importantly than that, 3 years is really not that much time to learn as much ophthalmology as you can and become a competent clinican and surgeon. Even at the "cusher" ophtho programs, you should be reading as much as you can outside of clinic hours, trying to get in on extra cases, suturing canalicular lacs, etc.
