We just completed a comparison of Mandrill and SendGrid here: indiemark.com/...
Ive used Send Grid for mingletrips.com. Works great. We are going to use Amazon SES since we are hosting the next app on EC2. Will give my feedback on SES.
Awesome service! Blows away the incumbents and has mobile monitoring apps! You MUST try it!
Rock solid. Mature APIs. SendGrid is the clear market leader.