Hiya,
I'm starting my camino tomorrow, and yesterday, at the last minute, I decided to buy a sleeping mat. I got a very compact one that weighs about 16 ounces/450 grams. Some people say don't bother with a sleeping mat, yet some people say that as you get closer to Santiago it's hard to find a bed, so I decided it was better to be safe than sorry. If I have to sleep outside or crash on a floor somewhere, at least I'll have some measure of comfort while I sleep. For me, feet and sleep are the most important aspects of the camino, so I've got foot care stuff and sleeping stuff. I guess it's all down to personal preference and what's most important to you, because everyone is different and everyone has a different opinion on what should be taken and what shouldn't. Sorry if that's not much help.
Buen Camino!
Annie