Hey guys,

So it looks like St. Croix has 4 different actions:

 

Extra Fast (XF)

Fast (F)

Moderate Fast (MF)

Moderate (M)

 

What's the MF? I don't fish SC often, so Moderate is for cranks, fast is for spinnerbaits? Or is their fast a bit faster than usual?

Which SC line are you looking at? I own quite a few Avid's and I don't think they offer any MF's in that lineup. 

Looking at the site, all three "Legend" lines  have a few rods with a mod-fast action. Mod-fast is a tweener so to speak between Moderate and Fast. There are a number on this forum that like the Mod fast for fishing spinnerbaits, jigs, crankbaits, among others. Myself, I like the mod-fast for my treble hook baits. I think a lot of guys like the mod-fast for that little extra give it has to help play fish and keep them buttoned up, but still is "fast" enough to get into the back bone of the rod quickly as compared to a true moderate action rod. Hope that makes a little sense. I am sure there is someone who will comment who can explain it better than I can.

Good explanation above, in my opinion.  One thing about St Croix is that their power ratings are usually a little conservative, meaning that their medium power will most likely be a little more powerful than most other rod ratings that are called medium power.  I think their action descriptions are pretty accurate and consistent with most people's expectations.

There are plenty of other manufactures out there that make mod-fast action rods as well. It's just a rod that's in between a moderate action and a fast action so the rod will load a little farther down in the rod blank. People like to use these for moving baits, as well as heavy cover situations (pitch/punch/flip). In the heavier cover situations some people feel they will help to spring the fish through the cover as they will load more on the hookset but maintain the power to drive the hook home.