I have really only used Mailchimp (and have had good experiences with them). However, there are some other interesting options out there such as ConstantContact, Aweber, Email Octopus, etc.

The main thing is that you need to find the service that matches your needs for the following as your business grows:

• Pricing
• Features / Functionality
• Ease of Use (usability)
• Integration (does it integrate to your shop, website, invoicing, etc.)

I use Vision6, Mailchimp, and now Zoho Campaigns. I think ease of use, mobile templates, and subscription form that is flexible are the most important features. I have had troubles with Mailchimp subscription forms but usually can get round it with a bit of trial and error.

I noticed that Zoho Campaigns has the ability to add a If This Then That workflow, so you can add "this set of data, e.g. postcode" then send this email or send to new segment.