Maybe I'm just playing a lot of iPhone games at the moment, but I kept expecting Killzone: Mercenary to ask me if I want to buy 500 Gems for £2.99.

The game is just driven by money. You get cash for kills (and more for headshots, triple kills, and other audacious moves), and finishing missions. But you lose money when you die, and must spend some to swap between weapons and equipment.

You also get to tackle most missions in any way you see fit - and you can take any weapons you want - as long as you have the money.

Guns for hire

You could spend your earnings on a sound-suppressed sniper rifle, for example. Most missions begin with the enemy unaware of your presence, giving you the opportunity to play stealthily, and take out foes with silenced weapons and sneaky melee moves.

If you stick with stealth you'll have to deal with fewer enemies, you'll pocket some extra cash, and you can interrogate wandering officers for bonus intel. But if you get seen and an alarm is raised, the game immediately turns into an all-out action FPS, and you're bombarded with extra enemies to tackle.

It's a good system, actually. You get to play stealthily if you like, but getting spotted doesn't result in a game over screen, or five minutes hiding under a box. You just get to play a fun shooter.

Your limited inventory does makes juggling the two playing styles difficult, though. If you're rocking a stealth set-up and you trip an alarm, you have to sprint back to a weapons box and change to more lethal weapons and more defensive armour.

Another day, another dollar

Those firefights are, at least, a lot of fun. They feel open and three-dimensional, which lets you keep moving about the battlefield, and even above or below enemies. The shooting feels satisfying, the guns are a joy to handle, and the different enemy types keep things fresh.

And, like every Killzone game, your character has a real heft and weight to his movements, which makes it feel like you're controlling a hulking great man who's carrying a rocket launcher and a bulletproof vest and rounds upon rounds of ammo.

But it's let down by the regular appearance of predictable missions we've done a million times before.

There's a bit where you guard a guy while he hacks a computer. There's an escort mission. An on-rails turret sequence. It's just missing the bit where you lose all your guns for a while.

There's also an infuriating focus on touchscreen nonsense. You have to stroke the screen every time you do a melee move, prod the screen to do an inane hacking mini-game, fiddle with the screen to set a bomb, and mush the screen to lob grenades or activate special equipment.

And the story is a lot of weapons grade nonsense. While you will - interestingly - fight alongside both the ISA (the good guys) and the Helghast (the evil space nazis), it's all completely preordained, and not as if you can decide to work for different sides of the war based on the pay cheque.

Pay day

Ultimately, Killzone: Mercenary is a fun and perfectly functional shooter, with a pretty typical single-player campaign.

I mean, it probably wouldn't stand out on PS3 - it just doesn't have that many fresh ideas, and having to jab your thumb into your TV to throw a grenade would be a turn off.

But by virtue of being on the Vita - the first handheld console that has a pair of thumbsticks, but somehow managed to monumentally mess up not one but two first-person shooters - it's a lot more exciting.

It's brilliant to be able to - finally - get the complete FPS experience on the go, and you lose very little in the transition to the smaller device.

Okay, so the teeny tiny analogue sticks don't offer the same freedom of movement as the controller, and a limited number of buttons puts sprint and crouch on the same button. But taken as a whole, it's a treat.

Cash for gold

Multiplayer is even better. This is a fully fledged online multiplayer mode, with various game types, different load-outs to play with, and some genuinely fun toys to help turn the tide of battle. Killzone's always had well-made and well-balanced online modes, and to be able to compete from your bed is just great.

Killzone: Mercenary is an enjoyable shooter, with an excellent online mode and a solid - if not completely predictable - single-player campaign. It looks outstanding on the Vita, and finally gives the little console the shooter it has been crying out for.