I've tried a few. 120gsm worked ok, but it was flimsy and warped when I rondoed it. That said, I made a decent looking chest piece out of it though I did need to do quite a lot of bondo work after to fix the warps. It utterly failed for a helmet though and the final thing came out really twisted when I rondoed it. It's great for fiddly High def models though, you can really get accurate folds since the tabs don't have much thickness at all.
Tried 300gsm too. That stuff's really tough but you have trouble folding fiddly high def models with it, your fingers also hurt after cutting out 30 sheets worth of shapes. My helmet's made from it, so it's also workable.
I'm now going with 220gsm, right in the middle to see what all the fuss is about!