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: 
There are definitely a myriad of options. I'm not quite as familiar with pricing, but ExactTarget did just get purchased by Salesforce, and provides an AppExchange app to keep track of your send results in Salesforce.

I have worked with Mailchimp and Mandrill with regards to Salesforce. While I think Mailchimp has released an AppExchange product in the last few months, the implementation that I worked on simply used the native webhooks available with Mailchimp and Mandrill, along with a custom REST Resource I wrote, where I retrieved all the email sends and created completed Task records against Contacts and Leads with matching email addresses. While you have to watch out for scale (the webhooks could send enormous amounts of data back if you are sending a ton of data), for a smaller company this is a viable option as it lets you do reporting on those records.
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.