Finished this up recently.
The book was very verbose, especially for something under 300 pages. Lots of descriptive text except where you'd really want it--like the X-Wing assault or the Duel in the Forest. I've always liked Foster's writing, but this go around seemed...off.
That said, there were some interesting additions to the story. Unkar Plutt and Rey have much more interaction than in the film. He seems like a creeper with an eye on Rey that reminded me of Jabba and Leia. After implying his intentions during the Jakku scenes, he and his goons follow our heroes (via the same tracker that Han used to find the Falcon which is, apparently, the same one Tarkin and Vader installed when it was aboard the ol' Death Star) to Maz's castle, where they try to nab Rey...
...but Chewie follows them in to rescue Rey, and rips Unkar Plutt's arm off in the process. This all happens between Finn leaving and Rey being called down to the vaults. Pretty neat addition, I thought.
By contrast, the scene of Poe getting off Jakku was pretty forgettable. The humor just doesn't play in the novel, and Poe spends the whole time joking, so it all falls flat.
We get more Resistance with Leia sending her envoy to the Republic capital to ask for more help...and it is through her that we witness the Starkiller's killer shot. Threepio is responsible for finding BB-8 and he reveals that he forgot to activate a tracker built into the droid. Instead he consults with the powered-dpwn Artoo and devises a plan to send out a galaxy-wide droid "Amber Alert" which explains why it is a droid at Maz's Castle that reports back to the Resistance when BB-8 shows up.
Snoke and Ren's relationship gets a bit of fleshing out as well. Snoke spells out the need for Ren to cut ties with Solo (pun intended) because it was Vader's emotional attachment that led to the Empire's defeat. Leia talks to Han about Snoke at depth, saying he had been watching their son for a while in his youth. To me this implies a relationship between Luke and Snoke before the rise of the First Order.
Ren's interrogation of Rey leads to his discovery of a mental block in her brain denying him access. Working against that is what allows her into his mind to discover his Vader obsession.
The infamous cut scene where Ren takes snowtroopers to the Falcon underwhelms in the book. I was expecting some reflection/introspection on Ren's part, but he just looks for people aboard and then gets distracted by the start of the X-Wing assault.
Ren expects the power of the Dark Side to increase in him when he deals with Solo, but he immediately feels weaker.
When Ren and Rey both summon Anakin's lightsaber and it goes to her, he exclaims "It is you!" but does not elaborate further. After Rey bests him, she resists the impulse to kill him, and recognizes that she is being called by the dark side.
Rey (and therefore us as well) witnesses Hux's shuttle landing and taking Ren away before Starkiller becomes a sun, which at least clears up the ambiguity of the movie. In his orders to Hux, Snoke agrees that Ren was "right about the girl." Again we don't get any elaboration on what that means exactly.
Rey finally meets Poe while randomly hugging him after the map is assembled.
Chewbacca insists that Rey sit in the Captain's chair on the Falcon when they ship out to get Luke. We don't get any kind of insight once we get there.
Like I said, the book is kind of a slog for how short it is, and IMO misses the beats of the action sequences and the crucial scene between Ren and Han. And the humor continually falls flat.