Very easy to override HTML with template overrides. Its worth spending a few moments of your life getting to know this extension.
It has a LOT of promise and delivers! Fantastic MVC coding, these guys obviously know what they're doing.
I don't know of any viable commercial alternative? No need I guess with this awesome extension.
This is not a replacement to Joomla! content items. Just a very elegant solution to formatting custom page types. One of the many features I love is the image field. It's not hard inserting an image into normal content items. But with K2 you upload one image, it has server side resizing, and has 5 image size options as configurable parameters. This is great should you wish to show a catalog layout or company profile page. You can set the dynamically created thumbnail to display on summery pages, and your large image on the full content items. Once configured it's as simple as writing the content, uploading your image and BAMM! it's done.
Great Open Source extension guys, thanks heaps for your hard work on this.
I look forward to learning more about extending K2.