I think Ascension is worthwhile for fans invested in the Star Wars EU but it's likely to be incomprehensible to those not immersed in the setting. The EU lost its way after the New Jedi Order book series and has never really found its way back. The Fate of the Jedi series isn't bad, per se, but it's meandering.
There's about forty plots in the book: including a group of rogue Imperials, the Lost Tribe making an alliance with Aboleth, and Daala's attempts to regain her position as head of a major galactic government. None of them are boring, except maybe the rogue Imperials, but they all compete for the reader's attention.
This is most true for the Lost Tribe of the Sith. The tribe, for those unfamiliar with it, is a group of a thousand or so darksiders recently freed from the planet Kesh. The problems with this premise have been discussed to death already on various forums.
Really, you'd think the Jedi would have noticed them in the past five thousand years. If not them, Palpatine or the other Sith. After all, the Emperor could sense Luke Skywalker gaining power in the Empire Strikes Back.
Still, the Lost Tribe of the Sith aren't your typical 'bwahaha' Sith and I appreciate that. Instead, they're more like the Sith version of the Kardashians. They're arrogant and beautiful snobs who just happen to use the Dark Side of the Force. I'm not sure how I feel about them since I expect my Sith to have a little more dignity than that.
The Lost Tribe of the Sith's members get about as much development as the Dark Jedi Revan kills by the hundreds in the Knights of the Old Republic game. Luke Skywalker kills thirty here, Ben Skywalker kills twenty there, and Abeloth enslaves a whole fleet of them at the start of this book.
Really, this isn't the lowest point the Sith have ever sunk to, but it's pretty close. I'm not a fan of the Rule of Two but these guys deserve better than to be used as lightsaber fodder for Luke and his son. The Lost Tribe of the Sith in Ascension exists for the sole purpose of making new villainess, Abeloth, look better. Given the Lost Tribe of the Sith could have easily carried the series themselves, I'm annoyed.
Abeloth, for new readers, is more or less the Star Wars version of Nyarlathotep. She's a shape-shifting tentacle monster with godlike power who was imprisoned by elder beings centuries ago. Luke and company accidentally let her out by blowing up Centerpoint Station and it's a rush to try and figure out how to put her back in. Either that or simply kill her, old-school style.
I don't mind Abeloth's concept, actually. I'm quite fond of the idea of writers creating new villains for our heroes to face than the old Sith and Imperial standbys. It's a 20,000 year old galaxy, why not create new villains capable of fighting the Jedi Grandmaster? It's just Abeloth isn't exactly the most well-characterized villain, befriending people only to suck out their brains. I'm still not entirely sure what her agenda is. With one book left in the series, that's not good.
There's entirely too much Abeloth stomping on the Lost Tribe of the Sith, especially towards the end. It's hard to take the Sith seriously when they spend whatever time they're not getting stomped on by Luke getting stomped on the Eldritch Abomination. After the events of this book, I'm afraid we're not going to see much more of the Lost Tribe and their schemes.
It seems a waste.
Luke, himself, gives a good showing in Ascension. One of the things the Fate of the Jedi series gets right is their treatment of the Jedi Grandmaster. Luke is brave, wise, kind, and heroic throughout. He's a bit more suspicious and vindictive towards the Sith than I expected, especially for a man whose defining feature as a Jedi is his belief no one is beyond redemption, but not to the point of being out-of-character.
Ben Skywalker continues to be inconsistently written. Part of this is the wholesale retconning of his character arc from Legacy of the Force. In LOTF, Ben was trained as a spy and as Darth Caedus' apprentice, becoming very canny and manipulative. He specialized in investigation with an almost Sherlock-Holmes ability to find clues. Now? He's just pretty much Anakin Solo 2.0, possessing a goofball personality and mad skills. I'm not complaining since I felt a fourteen year old super-spy was ludicrous even in Star Wars, but some hint of his prior characterization would have been appreciated.
One thing I did like was the treatment of the character Vestara Khai. Vestara Khai is a character that a lot of fans didn't know what to make of. For many, she was essentially Mara 2.0. A character that was raised by a Sith Lord to be an assassin and who had some romantic tension with a Skywalker. The book goes in an interesting direction with Vestara, managing a careful balance between a search for redemption and enlightened self-interest.
Vestara is a character I finally started to like at the end of the book and I'm glad they made her. I just wish that Luke had treated her better, since you'd think he'd be more forgiving of a sixteen year old raised in a twisted Dark Side environment. Likewise, you'd think he'd be trying to talk her out of the values that corrupted her.
The rest of the book is amusing and surprisingly light-hearted. I disliked the politics sections of the book, which unfortunately compromised nearly a third of the volume, but the humorous encounters with Han and the Squibs were worth at least half the book price. Squibs are always hilarious and Han playing the straight man worked wonders. Add in a decent showing by the rest of the Jedi Order and I wasn't dissatisfied with this volume.
In conclusion, Star Wars: Ascension is a fairly typical example of the Fate of the Jedi series. There's nothing particularly spectacular about the books but they have a good deal more "fun" to them than the NJO and LOTF series. Star Wars is meant to be a lighthearted tale about good vs. evil. There's room for moral ambiguity and tragedy within the story but good should ultimately overcome darkness. The Fate of the Jedi series remembers that and, because of that, I can't be too harsh on it.