I've been enjoying it since I was a teenager. Just the thought of it makes me incredibly horny. I guess it's just the erotic and forbidden nature of it, not to mention giving access to one of the most private parts of the body.
Although I didn't start out using toys, my first toy was this plastic vibrator that I enjoyed a lot. But ever since I discovered silicone toys, I've never thought about using plastic (or any other material) again.