I was wanting to work in at the gym one time, a LONG time ago, probably 9 or 10 years ago, on a Cybex curl machine. Wouldn't touch it now, but, at the time, it was my main exercise for biceps (everyone's gotta start somewhere).
Anyway, this douchebag was on the machine for about 5 minutes, just sitting there doing 1 set every 2 and a half minutes. I asked him if I could work in. His response, "I've only got 8 sets left. You'll just have to wait."
Ever since then, if someone wants to work in, I let them. Been that way through all the years. It's ridiculous to not let someone do a set in between yours unless it involves stripping 6 or 7 plates off the leg press or some shit like that. Only the BIGGEST DOUCHES at the gym don't let people work in, the same kind of guys that his poses in the mirror between sets.....