My company uses mailchimp for the sending of all our system related emails to customers, password resets, alerts the customer has configured, receipts, etc.
We have had a few issues with MailChimp in the past six months and they literally offer ZERO customer service. Well, they do offer email, but their response time is many hours. When you have a critical portion of your system (like email being sent to your customers) a response time of many hours is simply not acceptable.
On top of all that their prices went through the roof. We were spending almost nothing for this service and now we are spending about $1500 per year. When the price first jumped I thought, "Well things can only be free for so long." But the customer service did not change, improve, and frankly it seems to have gotten worse.
We will be moving away from Mailchimp ASAP.