I like it. Plastic plants are fun to aquascape with, and a great choice for fish that uproot or destroy real ones. The only way to make a nice crayfish tank that isn't hardscape-only is to use plastic.
The trick is finding good-looking ones. Pet stores aren't any help, you have to go to craft stores and repurpose their plastic plants.
EDIT: Also, nice plastic plants are the perfect gateway drug to full-on aquascaping. Learn the layout techniques, then add the real thing. I wish I'd done that when I started.