Soma goes well with just about anything, I've found. It adds more warmth and coziness to an opiate high (especially, I've found: hydrocodone, morphine, hydromorphone, or codeine when things are rough), with benzodiazepines it increases the muscle-relaxant effect of some (diazepam, etc), a wobbly-drunk feeling to others (alprazolam, clonazepam), but with the hypnotics (temazepam, halcion) it typically just makes me pass out really fast.
Believe it or not, I like taking it with my Desoxyn or Dexedrine as it takes the edge off in a way similar to how benzos do, but the muscle-relaxant effects make for no jaw-clenching and sore muscles. Same can be said for hallucinogens (mostly LSD, 2C-Xs, and DOxs), but only if you take a single 350mg dose. Otherwise it can mute the visuals and "dumb" down the trip.
Needless to say, I like the stuff.