I really loved the movie, so bear that in mind when considering this lackluster rating. Compared directly to the movie it's probably even only 2 stars. But between considering it as a standalone novel (which is difficult and somewhat pointless to do) and bearing in mind a few added insights, it holds up as fairly decent. 
 A significantly small portion of "added/deleted scenes" or "extended scenes" (or however we can consider them), as well as the inner thoughts and motivations of characters enhance the viewing of the movie. A couple of moments that made my eyes go wide. A couple that confirmed some assumptions I had, which also quell some baseless objections people had. Also greater detail in how the Starkiller base worked, and how the timing of the attack upon it was truly integral. Granted, I am by no means a physicist and I assume the explanation was a lot of hooey, but I found it interesting. 
 Most of the added stuff was where the book completely fell apart. For some reason about 90% of the additional dialogue were clumsy attempts at sarcasm that always fell painfully flat. This seemed especially true for Finn, whose entire character seemed to morph into a whiny brat that thinks he is very clever. But honestly it seemed no character was safe having a bucket of sarcasm dumped on them. 
 On the other hand, some of the funniest parts of the movies became strangely absent. Most notably when Poe and Finn first meet. A lot of this is surely due to the performance and chemistry between the actors, but the dialogue itself is altered much. The fast an electric camaraderie and banter between the two which subtly masked a layer of exposition was gone, replaced by a very clumsy, melodramatic, and mechanical info-dump. Peppered with Finn's newly discovered ability to whine sarcastically over and over and over. 
 Likewise there is a bizarrely different take on Rey returning to the salvage station with BB-8. (view spoiler)[Whereas in the movie the fat salvage dealer whops 60 portions on the counter and Rey briefly grabs for it, forgetting herself just for a moment at the sight of the ability to survive without worry, the book takes a different approach. He offers her 60 portions, and Rey haggles with the man. BB-8 whistles in objection, but Rey keeps at it, asking instead for 100 portions. The man agrees and then Rey decides not to. It is later explained that she was just..."joking"? All in all, it makes Rey come across as a very different sort of character than what so easily portrayed in the movie.... Unless my mind wandered for a moment in the movie and she really did behave that way LOL (hide spoiler)] 
 Aside from these complaints, another significant portion of added scenes were just utterly superfluous. Forgettable and bringing nothing important and unique. 
 I wonder if this book was based off of a semi-final draft of the screenplay, and on set between direction, editing, and chemistry, they really hammered things out where they became fluid and nearly perfect. As opposed to....this. 
 BUT, a fair majority of the book is a direct translation of the movie. And it mostly succeeds at bringing it across--failing only because of the stilted additional dialogue, wooden "added scenes", and losing some of the core of the main characters (both through a lack of having the performers add their unique cadence, and the questionable changes made). 
 I suppose novelizing a movie is not easy and probably rarely goes well. This was my first time trying one out because I was so hungry for added pieces of information from the movie. I don't think I'd do it again. 
 Overall it was not totally boring, when it stuck to what happened in the movie. There are BARELY some cool crumbs of info that you will not get from only watching the movie. This may be worth reading if you're a Star Wars nut with nothing else on your reading plate.