I couldn't tell you whether Raiders Of The Broken Planet is a better proposition now that the cooperative brawl-o-shooter has relaunched as free-to-play, but I can tell you that it's better-named: it now goes by Spacelords. What a name! Spacelords! I support all game names which sound like something I'd come up with, though I suppose we'd soon run out of Xlords, MurderXs, and X-o-Yers. The point is: all and sundry are now invited to bash in robots and moonmen, while maybe dodging a special enemy controlled by an invading fifth player.
Right so, you and your pals are shooting and smashing your through story missions, shooting from behind cover, refilling ammo by meleeing enemies, levelling up, unlocking stuff, and generally murdering in different ways with different characters. Most enemies are NPCs but a fifth player can pop up in games as the 'Antagonist' trying to foil them. It's made by MercurySteam, the studio best known for Castlevania: Lords Of Shadow.
I broadly heard lukewarm things about Raiders? Mostly from people who played Warframe instead. You tell me, gang. I'm just here for those wild-as-hell costumes. Those look like spacelords to me all right.
As well as adding a new story campaign, the update taking it F2P has reworked a lot of the progression and matchmaking to try to make it friendlier. See the patch notes for details on all that.
You can find Spacelords on Steam. The monetisation does sound a bit offputting, but it was always weird even when the game cost money to play. Weird. But, Spacelords!