Google's study on disk reliability indicates that there is a fairly strong correlation between some types of errors reported by SMART and increased likelihood of failure. The issue is that the likelihood of failure is still less than 50% - for example the average drive that reports a scan error is still 70% likely to keep working just fine for at least the next 8 months. So you're faced with the question of whether or not to replace a drive that ~probably~ won't fail.
Yes. I have been saved 2x with SMART warnings with some old Maxtor drives. Granted, I do have a very good back up system and these drives didn't have anything important, but nevertheless I was able to avoid down time by ordering some new HDDs before they completely failed.