I've seen those on HGTV. They are really cool. And they do really blend in well.
We like to entertain outside quite a bit in the summertime and this would be ideal for us. We usually cook on the grill and play horse shoes or whatever and we play the radio.
Here is a pic of the speakers...
Did you know that you can buy speakers that that look like outdoor planters? They are pretty neat too.
The costs for these different types of speakers vary. The watts vary as well. Also, they come in different shades to blend with whatever color stones you have in mind.
If you entertain a lot and don't want to drag out the stereo everytime you turn around then this would be ideal for you.