dolphin blue looked great, but was boring.
built on the metal slug formula, but lacking the soul.
Well, I don't know whether the game has found Jesus or not, but it's definitely not boring.

My guess is you played it on default difficulty (4). Metal Slug vets will probably be bored; I know when I first played it I was also underwhelmed. It's a fairly long game for a run-n-gun and on default doesn't get intense until stage 4 (and these are long stages).
It's really necessary to bump the difficulty up to 6,7, or 8. When you do, it plays like a new game. On 7-8, it's intense from stage 1. Bullet patterns/grenade patterns a lot more dense; bosses take a pounding instead of wilting away.
If you have a chance, give it another go on a harder difficulty (if you haven't already). I think you'll be surprised.