Important context: I read the GUNDAM SENTINEL RETRANSLATION version by Zeonic translations. That translation is based on the GUNDAM SENTINEL: ALICE’S CONFESSION version of sentinel.

The author, Masaya Takahashi, included this in the afterword and I'll abide his wishes and review this as a standalone novel.
"So I invite you, dear reader, to approach this work not as a quirky Gundam spin-off but as a standalone novel that happens to be draped in a familiar, beloved motif. Judge it on its own merits, and I'll consider my mission accomplished."

(Despite that, I would have to recommend having seen at least Zeta gundam if you plan to read this novel to appreciate the setting to its fullest)

Overview:

The novel centers on the conflict between the Earth-Supremacist New Desides and the Federation's Task force Alpha. Sentinel trades perspectives between both sides of the conflict with a few POVs on each side being focused on. I found the prose could be a bit dry, although a lot of translated works come off as that to me. There is also a lot of focus on technical detail, particularly in the first half.

I found the dry prose and (sometimes) heavy detail to be balanced out sufficiently by the fast paced plot and frequent action. There are also some lovely ascii art diagrams, that while sometimes a bit confusing, really added to the more grounded feeling. The technical detail (and just detail in general) make it really clear that everyone involved was quite passionate and put a lot of thought into things. (like, did we need to see the OOB of the Federation fleet? no, but it's cool to see).

The sides:

The two (primary) sides involved in the conflict are the New Desides and the Federation (through its Task Force Alpha). Both sides gain more support as the conflict goes on with a few more characters entering into the narrative as it goes on (and more leaving the narrative due to death or otherwise departing).

Unfortunately the factions in Sentinel suffered from an asymmetry of interest for me. I was generally just more interested in the New Desides side of things. That's not to say I disliked Task Force Alpha, I just didn't find them as engaging.

(I do wonder if I was perhaps biased with the knowledge that Sentinel is loosely based on Boshin War. The New Desides being inspired by/based on the Shinsengumi. As a fan of Gintama I do have a fondness for the Shinsengumi so that may have bled into my reading of Sentinel)

The Cast:

There are a lot of characters but only a few get many POV chapters. Most of which get given a small intro at the start of the novel.

Tosh Cray felt like arguably the main protagonist (or antagonist from the Fed side). He has probably the most substantial arc and developed character of any side of the conflict. It's his self delusion that largely seems to be the cause of this tragic war and it genuinely is tragic when

I do like the other characters we see on the New Desides side. Especially Josh and Brave (which is good, because they are the other main New Desides characters). The more minor characters in the New Desides or who interact with them (e.g. The Mayer of Ayers, their spy, characters from Axis) are also just more varied and entertaining then the Fed side.

On the Fed side, Ryu Roots was our protagonist. I wasn't the biggest fan of him at first, he's brash with a chip on his shoulder. This can make him come off a bit... annoying at times. He does grow on me a bit as the story goes on, as he himself is forced to grow by the war he's found himself in.

Some of the other characters on the fed side felt a little less developed. With the exception of Mannings and Heathrow.

Alice

The Action

I find book action in general can be a bit hit or miss. Sentinel definitely hits a lot more than it misses. From the arrival at Ayers all of the battles have a great sense of stakes to them. The fights are also generally fairly easy to understand and visualize. I was definitely more then satisfied by the mecha action provided by sentinel. (I did look at some images of the mobile suits involved while reading which did make it easier to visualize things. As a plus, I really like the Xeku Zwei)

Overall, very strong pacing.

Conclusion

Sentinel is definitely not for everyone, but if it is for you (e.g. a fan of mecha or military sci-fi in general) then I think it's definitely worth the read. I enjoyed my time with Sentinel and I've added it to my mental list of "things I would really like to get an anime adaptation".