People complained about the 6am email that by the time they got it all the the "today only" deals (that were posted the day before) weren't working the "today" that they got the deal.
2 sides to every coin. It all depends on whether you are trying to please the daily visitors or the occasional visitor.
The people who check the website daily will see the today only deals when they check it throughout the day. A 6 pm email summarizing the deals we've been looking at all day doesn't really help a daily visitor to the side.
On the other hand, a 6 AM email is perfect when most of the time the deals are added around midnight. That makes it really easy to see what deals were just posted.
Just MHO