What do you like best?
Nice error messages, good compliance with the standards.
Nice implementation of the standard library.
Compatibility with most features from GCC.
The sanitizers are very helpful, especially in the build farm.
Quick implementation of the newest features of the language.
The location of the error messages is often more accurate than those of GCC.
What do you dislike?
The generated code is not always on par with GCC: the code generator could use some improvements.
Sometimes the implementation of some features are incomplete. For instance, using return-type deduction was not compatible with the emission of debug symbols!
Recommendations to others considering the product:
On several regards, Clang is as mature as GCC. However, contrary to what people seem to believe, Clang does not make GCC obsolete. It is actually safer to use both, to ensure maximum portability. And often, the error messages of one are better than those of the other, and not always in the same direction.
What problems are you solving with the product? What benefits have you realized?
Design and implementation of a generic and fast library for automata and rational expressions. This is a heavily templates library, with template meta programming, on top of which bindings with Python are installed. Boost is used a lot.