I use AWeber. (Disclosure – the link is my AWeber Affiliate link). I got grandfathered in years ago with an account that lets me have a zillion subscribers for multiple accounts for less than $20/month.
In addition to the points @annliu made, I’ll add:
They have excellent customer service — real people answer the phone and they also have online chat help.
They haven’t done the best job in creating out-of-the box templates, and as a non-designer, non-programmer, I find their templates clunky and extremely slow to load.
But other than my issues with their templates, everything has worked like a dream with them.