UPS Surepost is delivered by the post office, after being sent through UPS shipping to your local post office, so it will be fine for anyone who gets post office mail (which should be virtually everyone in the US). I live in NW Washington state, and it takes Surepost packages a LONG time to get to me from the East coast, generally about 2 weeks. Sometimes even shipments from Seattle, which is roughly 100 miles from here, can take 4-5 days.
Also, the tracking isn't as complete or as reliable. When the package is first shipped, you track it through UPS. Then after it's transferred to your post office, you track it through the USPS site, using the same tracking number. I've noticed that the tracking seems to get dropped (no updates) quite often, and the package will show up at my house even though the last tracking info says it's at some far away state.
But I'm happy to have EF use it if it keeps the shipping free.
Bleu - Gary said we will be able to pay to upgrade.
Here are some details from UPS on Surepost: