I work for a company that has used them in the field, and the Engineers liked using them becuase they could write on teh paper, and then just transfer the stuff over to the PALM or PC that they wanted to save.
The only convenience of the Smart Pad is that it's so much easier to see outside. No screen--just paper. Although one guy took a whole day's notes without turning the Palm on and that was pretty funny actually.
Reliability I really can't talk about. I'm never outside so I wouldn't use one.