I paid $2 for a small (my local McDonald's is offering all small McCafe beverages for $2).
For the most part, the Shamrock Chocolate Chip Frappe tastes like a Chocolate Chip Frappe with peppermint.
Mine was thinner than a shake and more like a really cold drink. There weren't any ice crystals floating about texture-wise but there was the occasional speck of chocolate.
When it comes down to it, if you already have a fondness for McDonald's Chocolate Chip Frappe, you'll find much of the same in the Shamrock Chocolate Chip Frappe. You may be disappointed if you were hoping for something closer to the Shamrock Shake in terms of mintiness but I quite enjoyed the chocolate-mint combination.