Just yesterday my wife and I also switched from Sprint to AT&T. We both traded in our old phones for the $100 credit ($200 total) towards a new phone. I got the GS4 and she got the HTC One. With the credits we got both phones for about $226 out the door w/ a 2-year contract, which is less than what the upgrade price for the GS4 alone with Sprint is ($249). We lose the unlimited data, but we were only using 2.5-4 GB per month anyway, so whatever. So I think we made out pretty good.
If you're looking at taking advantage of this promotion I recommend you go to a company store. We went to an authorized retailer and they told us our phones were too old to get the full $100. When I questioned him he said we had to be within 3 years of the phone's release date to qualify. I understood that, but the Epic 4g (mine) was released in Aug 2010, and the Eco Shift (hers) was released in Jan 2011 making them both within the 3 year threshold. I didn't have that info handy at the time, unfortnately. But it wouldn't have been worth the energy to argue about it anyway. They were only going to give us $10 for mine and $12 for hers. So I pretty much said F U to the guy (in my head) and we left and went down the street to the company store where we had no problems at all. We didn't even mention the incident at the dealer store. They just said, "Yeah...we'll give you $100 for both of those." The whole deal only took about 30 minutes to complete. It really is as easy as the commercial makes it seem if you go to a company store. So I think the dealer was trying to bone us.
Nonetheless...both phones are awesome so we have no regrets as of yet.