Warframe has been on my "need to start playing" list for a year or two now. From the Steam player listing it looks like it has quite a few active players in game too
I highly recommend it EniGmA I haven't played in a year or two (and won't likely go back at the moment due to schedule) but it is 100% worth seeing for the first time and experiencing the game. If you have Twitch Prime, you can even get a free prime warframe and some stuff to start which is a nice place to be.
I have been playing it for around a year off and on now. It's become basically my fall back game after i get tired of another new game and it never ceases to amaze how much the dev team has done with a free to play game. To be honest, I would rate it my top free to play games of all time, excluding sub based games etc that went free 2 play afterwards. However if I include them it's easily in the top 2 if not still #1. I would say you should play it if for nothing else than the storyline quests, which by themselves really set it apart from other games imo. However, be warned, it has a high learning curve.