Online places blend together now for that sort of thing. I usually just check a few spots and triangulate the mood.
First stop is the Goodreads page for the book itself. You get the full spectrum there, from the superfans who pre-ordered and inhaled it in one night to the people who seem mad it wasn't the exact same book he wrote twenty years ago. I skip the five-star 'BEST EVER' and one-star 'NOT HIS BEST' extremes and read the three- and four-star ones. They tend to actually talk about the plot pacing and if the new protagonist works.
Amazon reviews are weirdly useful if you sort by 'most recent' and not 'most helpful.' The 'most helpful' ones are often from advanced copy readers and feel a bit polished. The recent ones are from people who just got it, and their raw reactions tell you more about the pacing and if the ending landed well.
For a more conversational take, I lurk on the Mitch Rapp subreddit. Someone usually kicks off a spoiler thread a few days after release. The discussions get granular—people arguing about tradecraft details or whether a character's decision was smart or just plot-convenient. It feels less like a review and more like eavesdropping on a book club, which I prefer.
My local bookstore's website sometimes has staff picks with mini-reviews, but for Flynn, it's hit or miss. Ended up just buying it based on the Reddit chatter last time.