I always try to have the signature dish when I can. I think it really showcases the talent of the chef to try the thing that they think is the best they have to offer.
But, I would also look at reviews and make sure that that really is the best they have to offer. I think sometimes chefs don't take the advice of the public and try to be unique and exotic, when really they should become more simple and more in touch with basic recipes.
If the signature dish has a string of flavors after it in the menu, it could be fabulous, but it could also be a sign that someone is trying too hard.
Looking at a lot of reviews of the place beforehand can be a safer bet than trusting the opinion of the chef.