(or, do they accurately convey messages from spirits/deity?)
For myself, at any rate, I find my readings for advice and insight to be fairly spot on the mark. With predictive readings, I have to admit I tend to be pretty bad about going back and comparing what I drew with what happened. I just stumbled across an earlier forecast spread (that I didn't type up here; I might go back and do that later) and I have to say, "Meh."
For me, accuracy seems to go hand-in-hand with spread size and scope. My short little 3- and 5-card spreads seem to hit the nail on the head, while larger spreads (10+ cards) get a bit lost. This is something I need to work on with my Tarot reading: processing more data and incorporating it all into a single, cohesive thought.