Aweber sets up fast and connects to just about anything. It integrates seamlessly with Wordpress to provide high quality opt-in forms for your Wordpress website.
Aweber is great for opt-ins using a standard form and for sending group emails. However, it has no pop-up or modal-style opt-ins and it doesn't host or create landing pages. For this reason, you'll need to combine it with a tool like LeadPages to do any kind of client work where you don't have access to the back end of the client's website.
Aweber seems to be working to add more functionality and become more of an automation platform rather than just an email and mailing list service. I used Aweber for nearly a decade and I still think that it's a sold solution. I would recommend it over ContstantContact or MailChimp. The platform is solid and reliable, that's huge in the marketing world.
I switched from Aweber to an all-in-one automation platform that contains a CRM, Email Automation, Landsing Pages, Sales Management, and more, because my company grew and my clients needed more functionality than Aweber could supply. They are a great company and if you are just getting into email marketing and automation, they are a very affordable start.
**A couple of things to consider are that they do not have a reseller program. If you want to use it for multiple clients, you'll need to run everything out of your account and separate it by lists. Also their mobile app isn't much help. You'll be running everything through the web interface. Finally, you should have a working knowledge of html coding if you're going to use it. You don't have to code to use it, but if you want to tweak the look and feel of it like a designer, you'll need to code to dial in what you want.
With Aweber, I was able to collect opt-ins through traditional forms in widgets and send things like email newsletters. Aweber will also connect to your blog or website to automatically send a notification to your mailing list when you post a new article.