I think some people like Vixskin better and some like O2. They're both good materials, but different. I'm in the Vixskin camp. I find Vixskin closer to the real thing in appearance, texture and firmness. The detailing on the Mustang is very realistic, as is the size (it's average girth and just a little longer than average). I think the curve is awesome. We also have Vixskin Johnny, which is well above average in size but similarly curved, detailed and textured.
We also have the Goodfella, which has the same girth and realism as Mustang, but shorter. It's pretty much exactly the size of an average penis. And it has realistic balls that sit outside the harness.
About half the time we use Johnny and most of the other times we use Mustang or Goodfella.