I used it for a while earlier at work, and don't remember many problems with it. One did have to apply OS patches fairly regularly to it, but IIRC, that process was somewhat smooth.
On HP-UX 10, back in 2000, the C compiler version I was using still wasn't fully ANSI C, and needed K&R C function declarations, but hey at least we had containers (HP Vault), and 64 bit file system access.