It's not the greatest example (since there are various ways to get SQL queries from a live mysql db - tcpdump/ngrep, query logging or strace from a client - you could strace the server but mysql appears to die when you strace it - dunno why).

Everyone who I know who has used dtrace loves it - so it seems likely to me that it is great+useful tech - but can someone come up with a real-world example of info it can give you that you can't get with tcpdump/strace?