Mercenaries is totally enjoyable for what it is; a vaguely shiny FPS on a handheld. I certainly have had some fun with it. That said, it's a pretty bad game by any other standard.
Serious question: What standard is that?
Graphics? Mercenary has some really excellent visuals.
Controls? Gameplay is smooth, and responsive, and pulling off headshots is a cinch.
Design? The level design is one of the things I enjoy the most about this game compared to other Killzone titles. The levels aren't gigantic, but they have a nice open feel to them, allowing you to approach your missions how you like. Compared to Killzone 1, 2, 3, and Liberation, I think Mercenary has the most enjoyable level/encounter design of them all. And I absolutely loved Killzone 2, Liberation, and 3.
Story? Yeah, story is minimal, but it is mercifully absent of Rico, excessive amounts of Testosterone, and, well, Rico again. Overall, the narrative is solid.
I can't think, for me, at least, of any other metric to judge games buy. If a game fails at those four pillars of game design, then, yeah, that's a problem. But I'm personally scratching my head at what other standards Killzone: Mercenary is failing at. From my time with it, it exceeds at every one of them, with story being the weakest of the bunch, and even then, the story was pretty interesting, if not overly complex/convoluted.
Again, not attacking, just wondering what you meant by "That said, it's a pretty bad game by any other standard."