If you sign up now (through tonight) you're in the beta forever, not just the weekend, so if you have any interest, do it.
I've been messing around with this all day. I really like it. The warframes are super-agile. I'm constantly sliding, flipping, wall running, clinging on walls, jump-shooting, etc. The melee is well-integrated, with different moves when you jump attack, slide attack, flip attack, and so on. Each 'frame also has 4 special abilities that you gradually unlock.
It is a lot like Vindictus, as mentioned above, with the key difference that the maps are randomly generated, which goes a long way preventing grind burnout. There are prefab rooms that are stuck together, sometimes with interesting results (a chasm that seems impassible, until you realize you must wall run along a pipe). I've replayed some of the same missions ~4 times and each time felt fresh.
I'm not sure how the F2P stuff is integrated, except there's a lot of stuff I can't access yet. Seems like a lot of XP grinding unless you're willing to pay, standard stuff I guess. Worth checking out for sure.