It's amazing for trawling through hundreds of thousands of lines of code looking for a complex pattern, a bug, bad style, or whatever that regex could never hope to find.
For example, I recently went through tens of megabytes(!) of stored procedures looking for transaction patterns that would be incompatible with read committed snapshot isolation.
I got an astonishing report out of Gemini Pro 3, it was absolutely spot on. Most other models barfed on this request, they got confused or started complaining about future maintainability issues, stylistic problems or whatever, no matter how carefully I prompted them to focus on the task at hand. (Gemini Pro 2.5 did okay too, but it missed a few issues and had a lot of false positives.)
Fixing RCSI incompatibilities in a large codebase used to be a Herculean task, effectively a no-go for most of my customers, now... eminently possible in a month or less, at the cost of maybe $1K in tokens.
Also, I found that I had to partially rewrite it for each "job", because requirements vary so wildly. For example, one customer had 200K lines of VBA code in an Access database, which is a non-trivial exercise to extract, parse, and cross-reference. Invoking AI turned out to be by far the simplest part of the whole process! It wasn't even worth the hassle of using the MS Agent Framework, I would have been better off with plain HTTPS REST API calls.