This is a series I really like.
The first book - The Lies of Locke Lamora - was excellent - absolutely brilliant. Just rollicking and gripping from start to finish.
In 'Red Seas', granted, it started somewhat slowly, but I'd argue that once they put to sea the book really took off, and became that cliché of a book that could not be put down. And, besides, I must say that I loved Zamira Drakasha - the pirate stuff and sea battles were outstanding. And Jean's relationship with Ezri Delmastro was wonderful.
Re Republic of Thieves, hm. I find myself sort of agreeing with you. While I love the backstories, - especially anything which includes Chains - I prefer them to have some sort of heft.
What could you not stand about Sabetha? Their relationship doesn't bother me, because I like the fact that Scott Lynch is capable of having a completely screwed up hero. And what are your thoughts about Locke's background?
--some spoilers to follow--
i found her very disappointing (maybe it was unavoidable) after the build-up in the first two books. but she is annoying as heck, very shallow and overall unlikeable. and i am not sure it was written to be like that on purpose. plus, to be this great love story there is no chemistry between the two.
as far as the revelations about locke origins, i think they detract a lot from his character, who goes from being a smart kickass nobody who raises on his own merits, to being the Secretely Predestined Hero™.
how cliche'. and totally unnecessary and out-of-tune with the rest of the story. i am guessing it is going to be central to the next book(s), but that completely changes the bases of the story.
i didn't like the finale because it renders the entire actions of the Gentleman Bastards completely pointless. they were already not very relevant because basically there was nothing at stake for them, except bragging rights vs sabetah, but even the final outcome of the contest, which could have been interesting to explore in the light of how the bondsmagi would have reacted to the forced tie, was voided but patience's massacre. and then you have the out-of-the-blue awakening of the falconer, which has no premises in the rest of the story. a deus-ex-machina with the sole intent to set up the next book
the writing is good, and some parts are clever, but the overall storyline -especially the 'present' one- is extremely weak