I think the problem would be the same for any hard mulch: Leaves and dirt will eventually cover it. It will stay wet and turn black. You still will be pulling weeds out of it even w/weed barrier.
However it's a cute idea if you have an "under the sea"theme going in a small space.

Like anything hard and crunchy, at some point when you have a shovel in your hand you'll regret you added it to your garden. I would also be concerned about the dyes they use for colorants although their ad says they are safe.

Probably a great product to discourage slugs though! If I could grow hostas here I might consider trying shells as a slug inhibitor!

>Probably a great product to discourage slugs though!

You would think, wouldn't you? But no, it doesn't seem to bother them at all, alas. I rented a place with shell mulch (natural, not colored) and that was the sluggiest place I've ever lived.

Last year, I installed a butterfly garden and received a lovely birdbath from my son to put into it. I thought a nice layer of colorful scallop shells would be a great idea for the bottom of this birdbath to give the butterflies a nice place to perch to get water. Didn't last long. They started turning green and black with mold, even though I refreshed the water every day, and my puppy also discovered that it was great fun to grab a mouthful and run all over the yard, dropping them everywhere and trying her best to crunch them up and swallow them. But it looked pretty for about a month. A very small theme garden would be very pretty if they were used as mulch, though. Since we bring back sacks full of them from Sanibel whenever we go there, at least they are free.

Is there a heat problem with shell, like you can have with rock or pebbles?