I've installed several and they don't look all that bad. The best looking one though was the one that I prepped and painted using a lacquer based semi flat interior paint. You can get the same effect using a good quality interior spray paint. Prep the cover using a clean scotchbrite and warm soapy water and then a different clean scotchbrite and denatured alcohol. Apply thin coats of color. If your old dash pad is cracked and swelled, it will need to be ground down or at least cut out the cracked, raised area. When installing one on an a/c car, I use the non a/c version covers and carefully mark out the a/c opening from the back side. I then install the a/c vent which will no longer be recessed. The reason I do this is because the a/c version never fits well around the vent. In my opinion, the vent area looks better this way. As was said previously, after glueing, place several heavy weights on top of the dash. I installed one in my 71 cuda 11 years ago and it still looks great. Most people are shocked when they find out that it's a cap.
I bought one of those aftermarket dash pads a few years ago and it looked so bad that I sent it back. When I want the car to be perfect, I send the pads to Just Dashes. If you send yours to them, don't forget to remove the vin plate or you will never see it again. They cannot return the vin plate to you for some reason. I say go the cap route for now. Much cheaper and and less hassle.