Take a look on shemaleyum. They have several preview pictures of post-op girls. They seem to vary from one to another probably based on the skill of the surgeon.

The biggest difference is lack of natural lubrication. A post-op uses lubracants to keep the vagina moist and wet during contact. I have dated post-ops that can actually have an orgasm vaginally.
Once you've mounted a "man-made" you will not notice a difference. Most skilled surgeons will size the vaginal opening for an average penis. There is a slight visual difference in the labia, but not enough to warrent distraction.
Your experience with a post-op will be driven by the person and not by the organ. If she is  hot and luvs sex you'll have a great time. If she was a bumb lay before the SRS she's still a bumb lay.