I use Aweber, I have a Getresponse account and I like them both. Great companies, features, and systems.
However, we were recently integrating a few more email list management systems including MailChimp, ConstantContact, Analytic Approach, and Send Pepper... I leave iContact off that list because their api is still in beta.
Out of all the systems the two worst to work with, from a programmatic standpoint are Constant Contact and iContact and the best to work with is MailChimp.
But beyond that... MailChimp is impressive in many ways. Clean site, clean api, clean documentation, clean UI, Can Do approach...
And have you seen their client base:
MailChimp Customers | MailChimp.com
You can easily assume that due to the caliber of client base that they have that delivery rates, accessibility, and flexibility are part of the package.
Since Aweber and Getresponse have both gone through price increases its also important to note that...
www.MailChimp.com costs about the same.
So when everyone is chatting about the two IM favorites... just remember there are other players out there that you can consider as well.