I've been using Mailchimp for my own projects and for most of my client work over the past 5 years. It's a solid platform, reliable, integrated with a lot of leading web services.
Before Mailchimp, I used to work with Madmimi, which was decent but not as advanced. I also tested Mailget for a while (not to be mistaken for Mailjet), a low cost solution to manage your lists and design your emails, but requiring a transactional email provider like Mandrill (now a Mailchimp company), Sendgrid, Amazon SES or Mailgun (Rackspace) to send your messages.
Recently a client introduced me to SendinBlue, a French company, offering a platform which includes newsletters management, transactional email and marketing automation (both via email & sms). It took me a while before taking the tough decision to move most of my lists and clients' lists to SendinBlue. But I'm very happy I finally did it. It was seamless. I just had to export the CSVs of my active users from Mailchimp (& remove all these users to reduce the size of my database before the next billing cycle), import them in SendinBlue, switch off the API integrations between my websites and Mailchimp (I'm using Bloom by Elegant Themes for most email capture forms) and re-connect the optin flow to SendinBlue, simply using my account name and API key. It took me under one hour to have everything up & running.
SendinBlue's pricing model is radically different from Mailchimp's. You don't pay for the amount of users in your lists (which can become very quickly quite expensive at Mailchimp). You simply pay for the amount of emails you want to send per month, with a daily limit in the free plan (100% free = max 9,000 emails per month, with a 300-per-day limit). Note: Mailjet has a similar pricing approach.
Let's say that you have 10,000 subscribers and send 1 email per week to your lists, i.e. 40K emails per month.
At Mailchimp, you would pay $75 (unlimited sends). At SendinBlue, $7.37. 10 times less! And your email quota also includes transactional emails, which have to be paid separately (using a Mandrill account) at Mailchimp, with a minimum of $20 per month for 25,000 emails.
SendinBlue also offers a very nice marketing automation tool, with multiple trigger possibilities + both email and SMS as outbound channels (SMS billed separately).
Check it out and tell me what you think about it.