Hello I got one of the first ones when they came out, they may be better now but here are the pros and cons.
Pros:
very rugged
easy to mount
easy to zero
no batteries
Cons:
Very hard to pick up(no where as fast as a red dot) really have to focus on the sight and take time to do it, as well as getting use to doing it.
works on a friction hold to your rail ( slides on rail and use a grub screw to push it up and away from the rail) and mars the rail
not very precise, you will not get the accuracy out of your airgun you would get with a different type of optic (scope/red dot)
The idea is great, small, light ,rugged, easy to mount and no batteries BUT I don't think it is perfected for the accuracy an airgun user knows and expects. With that said and you are looking at a zero magnification sight and don't want to be a slave to batteries look at a 1x prism scope (etched reticle always have a reticle, can use batteries and illumination if wanted). Vortex makes a nice on and I believe the gen ones were on sale for around $190. Hope this helps if you have any specific please ask Eric