I used SendGrid on a project last year. It was fine then. Are you sure you are getting a dedicated IP? I like the idea of using Amazon SES, but their interface is confusing as shit, and I don't think they have all the deliverability features of SG?
SendGrid dedicated IP's don't help. If all I wanted was an SMTP server, I'd install PMTA on the network and it'd be cheaper and easier.
"Hi, this is SendGrid support, how can I help you?"
"Yes, I'd like to get a dedicated IP and start sending 100,000 emails a day"
"Oh, you're going to have to warm the IP first"
"Okay, so how many emails a day, and how many days?"
"Lets see... 3,000/day for the first month?"
"So your service will queue my emails for me and make sure I'm only sending within the warmup limits right?"
"Well no, you need to do that"
"So you have pre-warmed IP's for me then, right?"
"Well no, you need to warm your own IP's"
"What is your service providing to me to improve deliverability?"
"Cold IP's, bounce management, and unsubscribe features, sir"
I don't get reports or anything useful from Sendgrid that justify there existence over a regular ESP. At least with Amazon you're getting pre-warmed IP's and instant high volume, even if you only send to your list once a month.
Also, compare your open rates. You'll find Sendgrid sucks compared to most of the competition.