Imagine unzipping the skin and putting a baseball there. Now have a professional ball player take a few whacks at that baseball a few times -- full speed. I know, it seems like such a great site; all large and easy to access. I know only one person that has done calves more than once, and he's definitely a masochist.
Pecs feel weird and are scary. (I actually just got brave enough to try this recently)
Biceps are okay, but can be crippling too if done in the short head of the biceps.
Quads are good once you have it down.
Lats scare me and I'm not flexible enough to do that without a stage 99 cramp popping up.

Traps scare the hell out of me as you could do some serious damage if you misjudge where you're putting that pin.
Glutes are gravy.
Ventroglutes are amazing and I agree that they are really underrated. Just a PITA to find if you're not sub-10% bf.
Delts are easy once you figure out how to stabilize your hand that's doing the work.
Triceps are okay, but I had a bad experience with them as the oil traveled down for some reason. Likely wasn't deep enough.
NEVER,
EVER pin the vastus medialis. (Teardrop of the quads) You will quickly learn where all those nerves are hiding.

Not crazy enough to try forearms, but I bet it's like the calf (gastrocnemius).
That's at least my experience with sites.
