Share 'Email Marketing Programs-What do you use?'

We are looking to get into Email Marketing and I've been assigned the task of finding a good program. I hear a lot about Constant Contact, does anyone use CC? If so, what do you think about their service? Next year, we are planning to send 5-6 email blasts that consists of 300k-500k people. So, we need a great provider that can handle the load.