I've had the Viper in .25 cal for 2 days now, and Im actually quite happy with the performance. After adjusting the hammerspring I managed to squeeze about 31J (23fpe) out of it using 28.24 grain H&N Hunter Extreme pellets.
I get that the price tag is quite the turnoff for a lot of folks. However, I do see where at least some parts of the pricetag comes from. When I put the Viper next to my Hatsan Sortie, which is pretty much its only competitor right now as far as semi-auto PCP pistols go, you do notice a huge difference in build quality. The Sortie feels really cheap in comparisson. The Viper is much more sturdy, compact, and does have a bit more power (albeit not by much). I won't argue that they couldn't have shaved a few hundred bucks off the price but.. it is what it is.
Im sure the Huben Pistol is gonna be a tough competitor when it launches, and may even result on a price drop on the Viper. But until then it's definitely the best of it's kind.
For me, around 30J is more than enough power to have a blast with it. Especially from such a small package. That said, Im a backyard plinker. I can't really say how viable this thing would be for pesting or hunting.