The new generation "Jeweled Plug" have a very good shape and the size is perfect for beginners as well as being really hot to look at. We bought a small stainless steel plug with the "diamond" crystal and it is the perfect size for my wife and a great visual when having sex from behind.
Good luck
Coming into this relatively old thread late, but butt plugs can be a real enhancement to play time if and only if there is a desire for anal play. Some people don't enjoy anything up their ass and some find it quite pleasurable and exciting. Butt plugs can be a good starting point or initiation to anal play above and beyond the "finger in the ass" which is probably the first step. They come in many diameters, hardnesses, and depths so it's easy to start out small and work up if desired. The vibrating kind can send super wonderful feelings in places you never realized you could feel them. Do some research and start with a bit of a "beginner's model" to test the waters.