I know this is an old post, but I actually find this combo quite enjoyable!
Definitely different from LSD alone, but if you're one of those folks for whom zolpidem doesn't produce confusion, disorientation, and amnesia at reasonable doses, the combo is pleasant and fun. I find that it reduces potential anxiety of LSD experiences, makes them more 'casual' and silly, but doesn't dull the effect so terribly. In fact, zolpidem is one of the nicer 'chill' combos with L that I've tried (GHB and gabapentin are also nice; opioids were sublime in concert with L but I haven't used them for years).
For me zolpidem feels like trippy benzos, but is far less dulling of psychedelic effects, and I don't find it to produce amnesia until about 30+ mg. For others, zolp. creates weird delirious hallucinations and lots of memory loss, in which case the combo (or zolp. alone) might not be so fun.