Overall I really liked the trilogy, and I think Stephenson managed to keep it moving in enough directions to keep it fresh and interesting to the end.

I had read his excellent Cryptonomicon - partly set during WWII, partly set in modern day Seattle and also a lot in present day Pacific area and Australia when it came out (the blurb looked to interesting to pass), and I was very surprised by this "prequel". Not the sort of direction I expected a writer of a sophisticated socio-techno thriller to take, but it was a very pleasant surprise - and it was good fun, and a good idea, to present many characters who are ancestors of those in Cryptonomicon (the Shaftoes, Eliza and most of all Watherhouse etc - and even more original to have a genuine recurring character in a "realistic" book like this). Some parts were a bit longuish, some parts like someone who've read too much Pepy's Journal or De Foe's book on the plage, or Braudel's trilogy on European early modern material life and economy (some stuff is even recognizably inspired by anecdotes and facts found notably in Braudel's) but I must credit Stephenson for making me care for the science and history of sciences in his novels. Not my cup of tea usually, but with him it was a problem at all and even lead me to start reading on the topic afterward.

I remember having had some reservations on his depiction of Louis XV's court and the French characters later on, but nothing book breaking, just an outsider's perspective, I guess, as he otherwise has used some facts from Louis XV's time about which most people who aren't historians know only generalities (details on Le Secret du Roy and what it was involved in, notably).

You might want to either wait or do some research on your own about Eliza's codes (Stephenson likes to make you work a bit, but then helps after a while). It's been a long time since I've read the books, but if I remember correctly, start with some basic documentation on Baroque cryptography (French and Italian, notably) and then jump quite ahead to find out the details of a code like hers, as Eliza's codes are intentionally quite ahead of her time (her descendants too will be crypto geniuses... both in WWII and during the recent techno boom) Stephenson is just going back further in the BQ in the history of cryptology - it's a major plot element of his previous novel, where the plot centered a lot on this. Her talent for codes continues to play a role in the plot of the BQ, and I think it's better explained later (you might even say you might not like Stephenson to go on too much at length about cryptography!)

A minor downside is that after a while those who've read a lot of history start seing him coming from afar. It gets pretty obvious what well-known developments, facts, events etc. he's going after, and what sort of fictional elements he might build around those events, but even then it never bored me.

It's probaly the contemporary historical novel that I found the most marking and fun since it came out, perhaps aside from Rita Monaldi and her husband's series (Imprimatur, Secretum, Veritas etc.), which was even more fun and also very well researched (a lot more about arts and politics - and secret agents again).