No, it isn't cheaper for everyone. I'm no fan of ATT but if I was going with them I'd be happy because most of my data is used on a WIFI network, and I never use texting (I use a separate app). That means for me, all I'd need to pay is the 39 bucks for 450 mins, which rollover if I dont use them all in a month, and then 15 bucks for 200meg of data (I checked my data usage for the past 6 months and the most I used was 45 off of 3g, with a ton more over wifi obviously). So for me, thats 55 bucks minus my employers discount (23 percent) plus the stupid taxes, which is a huge difference over 80 bucks for Sprint. I like that things are more ala carte at ATT. It prevents me from having to pay for things I have no interest in.
I'll be much happier when ATT gets a better selection of Android phones though.