I recently ordered a new magpul ms3 sling to replace my older ms2 sling that was pretty worn out.
When I received it I noticed the clips are now polymer/plastic.
I am also looking at their rail mount sling point. I believe they are made of the same material.
It feels pretty solid and I haven't had any problems the 2 times I had it at the range.

How durable are they in the long run? Has anyone ever heard of one breaking?

Have been using mine for about a year now, seeing medium use. The clip that attaches to the rifle in my setup (I run in as a one point most of the time) is the only clip that has some evidence of use. The polymer has been ground away slightly, but not even near enough to not hold in place. The hook istelf is actually metal so the weight of the rifle mostly sits on that and not the polymer. So far, no issues. I'll try to post a picture if I get a chance/if you're interested

Thanks, good to know. Mine will see medium use as well.

The photo would be good if it's not too much trouble.

Hopefully you can see it well enough in this picture. Again, seems just cosmetic at this point, alot more will have to come off before it won't hold anymore. My attachment mount is also sharper than the Magpul one, so that might make it wear faster as well.
(Might be a bit before you can see the photo, it has to be approved first since I have less than 50 posts)

<a href='http://i.imgur.com/MCRo6IG' title=''><img src='http://i.imgur.com/MCRo6IG.jpg' alt='' title='Hosted by imgur.com' /></a>

Thanks.

"Melonite finished steel and reinforced polymer hardware"   It's not all plastic...