I just picked up a M7 to go with my ONG folder. I don’t have any experince with bayonets. Will these take and hold a good edge? Or is it a waste of time to sharpen them?

Bayonets aren’t meant to have an edge that can slice paper.

It’s a stabbing weapon. Bayonets aren’t really knives.

You’d have to reprofile it and it probably won’t hold a good edge.

A bayonet of the M4 through m7 series will take an edge but won't make a really good cutting tool like a dedicated hunting knife.

Since these bayonets were developed out of the WWII M3 combat knife, the blades are the same.
You can definitely sharpen a bayonet just like the M3 could be sharpened, but it was intended to be a rough duty combat knife not a razor sharp cutting implement.

The M4 through M7 bayonets were intended to be both a bayonet and a combat knife, so as a combat knife the edge sharpening and holding abilities are acceptable.

I use a Gritomatic Hapstone sharpener. I have successfully sharpened 3 M7 and 3 M9 bayonets to hair popping sharp edges.
I realize they are bayonets but they are also knives. As a field knife they are far more likely to see use than they ever will be as a bayonet.
I am fixing to start a new M9 build that will be a full tang made out of O1 tool steel.
It will feature a very nice hollow grind with an amazing edge. If the day ever comes where I need to affix bayonets, I am sure it will long outlast me!