I am not an expert but this is what I know.
Simple mobile has nationwide coverage using Tmobiles network. You have access to Tmobile 3G and thire 4G service with the proper equipment. As far as i know any 3g mobile phone will work. I use a samsung vibrant and i get the same quality of service as any other customer on tmobiles network.
Unlimited talk + Unlimited Text + Unlimited International Text(http://www.mysimplemobile.com/Simple-Mobile-ILD-Text.aspx
Everything in $40 plan + 100Mb of data(for use with web surfing or picture messages/mms)
Everything in $40 plan + Unlimited Data
You need either a Tmobile phone or an unlocked GSM phone. As far as i know the #g/4G only works with Tmobile 3G/4G phones. I can confirm through personal use that the Samsung Vibrant, MyTouch 3G Slide, MyTouch 4G and the Tmobile G2X all work as expected.
I personally use the $60 plan, I made the switch from Tmobile to Simple and haven't noticed any difference in the service, But I have noticed the lower price!