As a side sleeper, my opinion is that the z-lite is crap for side sleepers. I tried it for two nights, then promptly put it in the car for picnic and bleacher seating. Luckily I picked it up on freecycle so I wasn't out any money. The problem is the eggcrate pattern flattens out. For a back or stomach sleeper I think this is minimized b/c of the larger surface area of the body pressing down. For side sleepers all that weight is concentrated on two points and it feels (to me) like there isn't any padding at all under my hip.
That said, underneath a second pad I suspect it'd be fine. But by itself I wouldn't recommend it unless you're acclimated to a very hard sleep surface, or else only camp where there's soft ground.
I recently picked up a solite for a song and have only lain on it indoors so far. My observation is that it's not any softer than the zlite, but it doesn't flatten out, so what padding there is actually serves to cushion.