From a developer viewpoint, I like the design concern behind Solaris which is lacking in Linux. As a by-product backward compatibility is a great advantage over Linux for "stability in time" and avoid wasting time rewriting things or breaking drivers every minor version like I experienced with Debian.
That's maybe why OpenSolaris evolves quickly: I think it's ahead of Linux in term of design but I admit used to lack sometimes some facilities for a GNU newcomer and hardware support (some USB peripherals,Radeon chips). But overall driver quality seems better than Linux (Wifi, Ethernet, USB framework, audio => I dislike PulseAudio and ALSA).