Like it was said, primary students are fun, but they are a challenge. I teach at a university so we get the entire strata of students with all their baggage too. You will be a better stick and rudder skill guy and also will begin to see what your first CFI was saying when he talked about using the rudder and feeling the plane. Usually, we subconsciously develop these skills, but when you fly with someone who doesn't have them, it becomes painfully obvious and lets you know what to look out for.

Sidenote: In previous semesters, many of the new-hire CFIs at our school did not get student pilots because flying with a commercial student has skills that are recent to you and also allows you to get your feet wet without the high responsibility for a student pilot.

Instrument students are interesting and are in many ways like student pilots. Early on, you will have many times to wonder "Where is he going? I said turn to ____________, or hold ____________!" It can be frustrating, but in the end, when you do the cross-country flights, you can really enjoy it (and seek out IMC). Just make sure they study a lot. Another benefit: the training is pretty much all dual, like primaries.

I haven't had any Commercial students, and I feel for any I get (jk). Personally, I think it's very reasonable for the student to be able to hold altitude, heading, and airspeed on the dot since they are fresh out of Instrument. Also, they need to have a much better decision-making ability than say a student pilot. After all, they are a PPL with Instrument priveleges. I should NEVER have to ask them to check weather, W&B, or preflight. Those are the things you do anyway no matter what you fly. Obviously, you can't have those kind of standards when they first begin practicing the new maneuvers, but for PPL maneuvers (straight and level, climbs, turns, descents, steep turns, and stalls) there isn't much excuse for not meeting at least Private PTS.