I have used the Cokin P series ND soft grads for the last year. More often than not, it's the 0.6 that I use (2 stop) and I have never stacked them as I use a wide angle holder that only has one slot.
If there is a huge amount of contrast, I may use the three stop and hardly use the one stop as a small amount of recovery can be done in Photoshop although obviously allways better to get it right in camera.
I've had no problems with the filters, but I'm most skilled at breaking the holders and also bending the filter adapters out of shape. I have recently bought a Lee wide angle filter holder and 77mm adaptor, just not got round to buying any filters for it yet.