We're a Knak customer as well and have been very satisfied with their library of templates. Their templates for landing pages and emails are easy to customize and very well designed. The templates work great out the box but if you need to make a few modifications, it's pretty easy. I'm by no means a front end developer but I know enough about HTML/CSS and bootstrap framework to easily modify their templates for our needs. I would echo Drew's post by saying their customer service is great and they are very responsive. Pierce Ujjainwalla has walked me through some Marketo setups and definitely learned a thing or two from him.
Seen a glimpse of what they have in the development pipeline and it's some cool stuff.
I've also been using Knak for the past few months and have been extremely satisfied with them. They have a good variety of email and landing page templates and if I need to modify them I can do so with basic HTML/CSS knowledge. What I love most about Knak is that if there is something I can't modify myself they're there to help me. I've even downloaded a template with the wrong social media icon and they fixed it and emailed it to me before I even noticed the error.
I'd say stick with Knak as they're growing fast and open to developing products that will work for you.
We're expertise in creating responsive email templates. Here's a link that provide you with the email templates galore from where you can import various email templates and all of them mobile friendly.
Hope you find what you're looking for!!