This card already has 3 reviews, but I want to add my two cents.
A major point nobody here brought up when comparing it to Mimic, is that Mimic is a fixed strength breaker. This is very relevant, as shapers are the faction which plays Self-modifying Code. This card allows them a lot of the aggression they usually lack. Just install an SMC and run happily with the knowledge you can just fetch any breaker you want to deal with the ice the corp rezzes.
Unfortunately, Sentries tend to be the worst type of face checks. Running head first into an Archer, Colossus, Ichi 1.0 or Cortex Lock could potentially set you back enormously or even lose you the game. And Mimic can't break these sentries unless you already loaded up your Datasucker, which again requires more support. You could tutor for Mimic before you start running AND install Dedicated Processor on it but that also takes up time.
This is where Na'Not'K comes in. Sure, the boosting is kinda awkward. But here's the thing, you can boost. And it allows you to deal with any sentry, provided you have the cash. This makes it ideal for big rig shaper decks like Kate, who is strong late game but suffers from lack of early game pressure. With this breaker you can keep the corp on their toes, while waiting for a Dedicated Processor to make it more efficient.
tl;d: It does not compare favorably to Mimic in the credit department, but the ability to tutor it with SMC and boost it makes it worth it.