I have used Campaign Monitor for the past 3+ years.  Before Campaign Monitor I used various other providers (ExactTarget, SilverPop, MailChimp) as well as some homegrown solutions.

Please don’t ever use Outlook or regular email to send out a newsletter to more than 10 people.  The risks are not worth the tiny amounts of money you “save”.

Why I use Campaign Monitor

1. Easy to add signup forms to your site.
2. It’s cheap -  $5 per send and 1¢ per recipient
3. They have really nice reports - the reporting is similar to Google Analytics
4. They have built in auto-scrubbing of lists.  Old emails are automatically cleaned out of your lists.
5. Emails are easy to customize - I can either use my own html/images or pick one of their existing templates
6. There is free preview & testing of emails.  Emails are hard to get right across all the different clients (don’t even get me started on lotus notes )
7. There service is reliable.  I haven’t had any issues in the 3+ years I have used their service.
8. They are experts in their industry.  Their blog is top-notch.  They often post research & tips that really and truly help.  For example, check out this post about CSS support in email clients.
9. They have a powerful API. 
10. They have serious reputable clients (this can help if you ever need to justify a decision to someone).