We've been pretty lucky with sound guys on the whole. There have been a couple of notable exceptions, but I tend to work on the basis that having a go at them serves no useful purpose - if they're crap anyway, giving them a bollocking won't make them any better and they'll stop trying on your behalf. I just don't generally bother to thank them from the stage. I must stress, however, that such experiences have been few and far between, and it has to be a pretty shite and insensitive mix to piss us off!