Let's get this out of the way first: It isn't perfect.
Do I still recommend it? Absolutely.
For $15 you get a interesting story that takes two old enemies from the "prime" Star Trek universe/timeline and twisted them up into one new fearsome foe threathening the entire galaxy of this Star Trek alternate-timeline. Add to this story heavily borrowed mechanics from games such as Deus Ex Human Revolution, Batman: Arkham Asylum/City and the Gear of War series and you have solid gameplay as well.
Your choice of playing as either Kirk or Spock won't matter much wether you play solo or co-op. The main difference is in the standard/primary weapon that you use, with Kirk having mainly his captain's phaser pistol and Spock having a Vulcan repeater by default. Both work slightly different buth are capable of the same result (to kill or stun).
All the characters are voiced by the actual actors that reprised these characters in the movies. Though the graphics might make their heads look a little on the strange side, it isn't game crushing.
The other weapons available to Kirk and Spock all have their own flavour of primary and secondary fire and add up to the gameplay tactically.
The ever-present companion through the missions is the Tricorder. It acts the same as augmented-vision or "detective mode" (from the Batman games) allowing Kirk or Spock to notice hidden circuitry, detect life-signs beyond solid walls, interact with mechanisms at a distance and even distract or sabotage the enemy or head and powerup each other.
The main borrowed mechanic from Gears of War is the "downed" state when either Kirk or Spock get knocked out to an inch of their life. The other can quickly come to the rescue by tapping a button in a timed sequence to just heal them up or even supercharge them whilst healing them up.
The other GoW mechanic is the ubiquitous "cover" mechanic, whereby you can take cover behind ye olde Chest-High Cover(tm) allowing you to blind fire or take quick aimed shots whilst temporarily stepping out of cover and then back.
The similarity with the Deus Ex games comes from the stealth elements whereby you can take several approaches to completing objectives, including alternate paths and the ability to chose whether to take a forced aggressive approach or stay entirely stealthy. There will be crawling through "jeffries tubes" and vents. The stealthy approach is expressedly rewarded in most of the missions.
The reward for completing objectives, and in a lot of cases completing them stealthely (you get a Commendation) is experience that can be spent upgrading/modifying your Tricorder and Kirk's or Spock's primary weapon. Although you can upgrade the other character's equipment whilst playing solo, it won't be used by the A.I. (the upgrade) despite being able to buy it and "install" it.
About the AI...This is a movie game, with a focus on two player Co-Op. This means that its NOT the best game due to the usual rush to get the product out the door in time for the movie, etc. There are glitches, they are minor and haven't really stopped me from finish the game in a day of non-stop play. Your AI controlled character will come to your aid and will follow you, teleporting to you if they get stuck (which they do sometimes) however in Boss fights (and there are a few) they might end up getting downed due to standing still where they ought to have dodged. I HIGHLY RECOMMEND not trying to resurrect/heal them if you can or focus on keeping the Boss's attention on you by constantly firing and running.
This is a minor grievance and I honestly expected FAR worse from a movie game. They are historically known to be horrible messes. This one is actually quite good.
Lastly, the involvment between the characters is a bit cliche, but don't let that distract you from the real stars of the story - the enemy you are facing.