Question: We'd like to log all transactional emails within a customers Salesforce account. I'm looking at things like Mandrill (as marketing is already using Mailchimp) and SendGrid.

Before jumping into this more and using Mandrill simply because we have our foot half in there already, does anyone have any experience with Salesforce compatible email platforms?

We're looking to send emails, log transactions back to Salesforce, and do some fancy reporting.

Answer: 
Since no one has yet weighed in with a sendgrid opinion, I thought I'd throw in my 2c.

There is already a sendgrid integration package that you can purchase called mass mailer. [MassMailer AppExchange Link](https://appexchange.salesforce.com/listingDetail?listingId=a0N30000005uxj5EAA)

This makes it pretty trivial to use sendgrid, and with sendgrid there's no warming or reputation building as there is with mandril.
We've been using mandrill in production and have been very happy with it. We use a custom object and webhooks. Works great. Let me know if you need tips on implementing. It took a fair amount of custom apex.