Receivers
TV
Switchers
Controlable Amplifiers
etc...
These are usually much better via 232.

If it has a direct input on the back I use it (IR or 232). I have done many racks where the only emitters used are on the cable box(es).

Every manufacturer is different. Denon for example does not allow unit ID control via 232 (yet) like they do with IR. It depends on your need and your level of familiarity with the components.

With 232 you have absolute control over the comand strings. With IR you do not have this same absolute control unless you are generating your codes cleanly. I try and generate all of my IR for reliability so for me there is no time difference when using 232. I would rather shoot myself than have to consistantly rely on learned codes (mine or from others).