The Vortex Viper HD has a very neat feature, it has a short piece of Weaver rail attached to it. You can use a cheap pair of Weaver compatible Chinese QD rings and a cheap pistol scope to line the scope up with, or better yet something like an IER scope and all of the arguments about lining THAT particular angled scope up and are nothing but hooey.
I don't own one, but I have used one in the field that was set up with an old Burris IER scope and what I can say about that setup is: I sure wish my scopes had that feature.
Unless you are looking down a lot, angled scopes can be worth consideration.