I think I've used all of them at some point during a mix, and loved them for what they did. They are not only great at performing their task, but the ones designed for having character bring some very unique and usable flavours to the table. Thrillseeker, Ferric and some of the colour EQs come to mind, but I'm probably forgetting some. I'm fond of Density too.
Ferric is one of the very few plugins where I use the same setting very often, because it always seems to do the trick.
I set both the saturation and the dynamics to about 25% (this is on properly gain staged tracks) and correct the trim to get unity with the plugin on bypass. What this does for me is that it makes tracks pop out just a bit more, while retaining their character. Similar to having parallel distortion on something, but more subtle. I really like it when I want vocals or some kind of lead to stand out a bit more, while still keeping them embedded in the mix.