I don't think penis tattoos are all that common, although the concept is finding a home in a small population of men.
I've actually recently become fascinated with the idea of getting a penis tattoo. I agree with some people that some of these don't exactly look good though. However, I think if we apply the concept of "less is more", there are some ideas that actually end up pretty cool. And, ultimately, everyone's tastes are different and the tattoo isn't necessarily for anyone else besides the owner anyway.
Regardless, I'd be more interested to know the process and how difficult it was to find someone willing to do it. Not only willing to do it, actually, but also willing to not charge an arm and a leg for "handling fees" as I've seen them called.