A nice matte black finish, solid build, very sturdy. They aren't going to break in a hurry

Hinged handcuffs are good, they make picking a lot harder, and depending on your flexibility, getting them from behind your back a lot more challenging.
Additionally, if you have the cuffs on tightly and the keyway is facing down 'away' from your hands, it becomes significantly harder to pick them as finding a suitable angle with a tool becomes extra difficult.
The M300-1 has an unusual 'double-locking' feature, what appears to be a 'wire' in a gate as shown here

Which disappears when used (and is a handy visual indicator)

The bow comes to a fairly blunt point

And is more like the Peerless 700C than the Peerless 500's in terms of shape and size

From Left to right there we have the S&W M300-1, Peerless 700C and Peerless 500's

The above image also shows a comparison of the three aforementioned bows.
Unfortunately, the S&W's have the same single ratchet design as the Peerless cuffs with three teeth as shown here:

They also have the same wide ratchet which means you should be able to shim down the side (the tolerances between the edge of the frame and the bow are pretty tight compared to the peerless models). Shimming along the bow track however is still easier (even more so if you can apply another notch while pushing the shim in).
In terms of keyway size they're somewhere in-between the dimensions of the peerless 500's than to the 700C's which means that its possible to get most tools into the keyway without much fuss if you wiggle a bit.

It seems that the S&W's are similar to their Peerless peers. There's no additional security of note beyond the standard basics.
So in summary:
Shimmable?
- Yes.
Easier to shim than the peerless cuffs. Tension wrench or broken picks work fine.
If you can get something thin enough down the side that's also good. Make sure they dont get jammed!
Pickable?
- Yes.
As easy to pick as any of the peerless models previously described.
Double locking?
- Yes.
Easily un-done when picking.
A couple of strong knocks against the ground and the double lock has not changed state.
Join?
- Hinged. Very strong. Effective at restraining movement, especially if the cuffs are on tight.
Other vulnerabilities?
- None noted.
Any Type of additional or unique security?
- No.
A note for anyone who's not terribly familiar with handcuffs, the S&W handcuff keys are widely regarded as 'universal' or 'master' keys.

They fit every pair of handcuffs I own currently (with one exception which I'll explain in more detail in a later post).
It's worth buying a few of these keys if you play with handcuffs, and its also worth keeping one handy (mine is worn around my neck on a chain) at all times when playing with cuffs.
I'm fairly sure its possible to buy these keys independently of the cuffs as well if you are so inclined.