Apparently they have made some changes to the cartridge that will allow it to work in more pistols AND rumor has it that somebody is making a drop in Glock conversion barrel! I'm excited about this.
It's essentially a cut down 223 case necked down to hold a 40gr bullet at around 2100fps. Maybe an alternative to 5.7? I have always kinda dreamed of having a G19 in 5.7 for fun but of course that's impossible. This might be the closest thing.

Any opinions on this round?

I have one of the RIA TCM combos.  I bought it because I wanted the 9mm part, and after owning I feel the same way.  The TCM goes bang and tosses lead at high velocity, beyond that it's just another caliber we don't need IMO.

The last thing I need at this time is a wildcat cartridge.

I'll stick with the 5.7 pistol, Elite Ammunition S4M is listed at 2,600 fps out of the 5.7 pistol.

The one thing I think I'm going to do is have Elite do a trigger job on my 5.7. Supposedly they get a crisp 3.5-4 lb trigger out of it.

There are way too many positives on the FN 5.7 pistol

Positives:

Uber Light (it weighs less loaded than my Glock 17 does empty)
It holds 21 rounds (31 w/mag extension)
I really like where the safety is located
Reversible mag release
Has an effective loaded chamber indicator
Almost zero recoil
100 yard capable
EXTREMELY Accurate
Easy to clean
Hard chromed chamber and bore

Cons:

Fugly
Cost
Crappy factory trigger
No aftermarket sights (tritium can be added to factory sights by several companies)
Limited holster selection

TCM is enjoyable to shoot. Zero recoil in a 1911.

I guess guys have been buying the bolt rifles in TCM and rebarreling them to 9mm and shooting subs. Seems like a better use of a rifle to me...