...and it was awesome.  The game is easy of course (except for the True Arena), but nobody really plays Kirby games for a challenge, it's all about the experience, which here gets almost cinematic at times--half the fun is in watching boss battles unfold as the bosses progressively change up their attack patterns throughout the fight.  And the final boss... well, it was something else, let me just say.  As someone who grew up playing Kirby Super Star it must be seen to be believed.

It looked pretty awesome. The mechs at once remind me of the Robot Ride Armors from the SNES Megaman X games and the combining powers mechanic from Crystal Shards.

Can you play the main game multiplayer? That's like, 90% of the fun of Kirby games (and what makes Super Star on the SNES so good).

If you want an idea of how the mechs work in gameplay, just take the Ride Armor from Megaman X, design the levels around it, then imagine what that would have been like if you could have used powered-up versions of Megaman's abilities (that he gets from beating bosses) while riding it.  That's basically what it is.

Besides the inspirations you mentioned, the game also apparently takes some inspiration from Gurren Lagann, an anime I've never seen but now want to thanks to this game.  Particularly the design of the mech (it has a face [Kirby's] on the front), and one section of the final battle.  As for multiplayer, Return to Dream Land on the Wii had 4-player co-op (not surprising, being a Wii game), but that feature hasn't returned since, sadly.

Even without the multiplayer though, I'd definitely recommend picking up the game if you can, especially since you mention liking Super Star.  Without spoiling anything (and trust me on this, I'm pretty casual about Kirby spoilers, but this is worth going in blind for), the whole final battle will blow you away.

Oh man, I can't believe I didn't think of the Gurren Laggan connection. They even showed off the drill-armed robot in the initial trailer.

There's Smash Kirby in that too, right? Does he actually literally control like he does in the Smash games?

Yeah, the Smash ability controls pretty much just like Kirby does in the Smash games, with all the same moves, etc.  The physics are a bit different of course (different engines), but otherwise it's identical.