Can't stand 24s. Sure, I enjoy the fact that if you take a single shift off, you're off for 5 days (Hell, I've taken the last 2 shifts off, which means I've been off for 8)
But:
You can be up for 24 hours straight
If you're up all night at work, all you want to go is go home and sleep, BUT if you sleep, not only do you waste one of your days off, but you also cannot fall asleep till late the next morning again.
You get one day of 'resting/fun' and the next you have to be in bed at a decent time because you have another 24hr shift the next day.
I much prefer 12s, especially 3on/4off, 4on/3off. GO home every day, sleep uninterrupted in my bed, no being held late if your relief doesn't show up, you KNOW that your shift is ~12 hours no matter what, even with a late call, and not 24 with the possibility of 36+.
Yea, give me 12s. Heck, even 16s sometimes.