Rangefinders are awesome. I don't use them, as I rely upon calibrating my parallax wheel on my scope for rangefinding. I dislike carrying something additional into the woods (to lose).
I have the Halo 450xl. It works just fine for me. I would have to say that it is very accurate and all that is needed for air rifle use. No need to buy a high price range finder unless you plan on using it for powder burners.