It's not overly rare as long as you are able to get chest engrams from three of coins. I wouldn't really view any of the exotics you can get from engrams as rare at this point thanks to them. As for usefulness. It's a lot of fun and is pretty good for getting out of bad situations or making quick approaches for shot gunning in pvp. There have also been some good videos of people managing to dodge supers and rockets that are awesome but it would take some getting used to to really pull off well and hasn't been overly used for that. In pve it's less useful but is still good when u need to get away from an aoe attack quickly or want to get in and out quickly with a sword or shotty.