It's the story of a recent interaction with a client that went sour because I didn't stick to my guns about using premium themes. Many premium themes are excellent and well-coded. They don't conflict with plugins or otherwise make life difficult for web designers. Most of the time you don't even need to make changes to the colours and backgrounds, etc. but sometimes the job demands it.
There are free premium themes. Basically, for many people it's about whether or not you have to pay for them. For me, it's about whether or not they work as intended and allow me to do whatever I want without having to hack them. The trouble with hacking themes becomes apparent when you update and the old settings are put back in place. A good premium theme, to my mind, allows you to make whatever changes you want to the colours and backgrounds without limiting your ability to do so.
How can you tell whether a premium theme is any good?
The demo isn't much of an indicator. Personally I like plenty of widget areas and good interoperability with a variety of plugins. Over-coded themes can cause conflicts, as can badly-coded ones. One indicator of whether or not a theme is any good is the support. Another is the maker's ability to operate in English. A poor grasp of communicating in English makes me worry about his or her ability to code in English. If the code is not in English I can't work with it.
Building with premium themes
Ideally, you ought to build from the content out and choose the theme accordingly. If you don't, you may find yourself having to rearrange the content to fit the theme whether or not this is a good choice from the point of view of presenting the content to its best advantage. If you've got a fixed idea of what you want your website to look like, you're better off choosing the plainest theme you can get and working from there. Otherwise you'll have to overwrite the CSS in the little customization box they give you and sometimes the customizations you can make are very limited. It goes without saying that the more gussied up a theme is, the harder it is to make changes to it because the theme author will be all the more precious about it. Bear also in mind that if the theme doesn't specify whether or not it's responsive or otherwise mobile-ready, it's probably not.
How do you make the right choice?
I've said it till I'm blue in the face; build from the content out. Be guided by your content and your message. That way, choosing the right theme is intuitive, based on what you want to present and how you want to present it. If you try to build your content and message around your theme you're going to have all kinds of things go wrong for you. Over-coded themes have their place if you're not planning to do much with the site and the development is already done for you. It's when you want to use an already code-laden theme for more complex work that the problems start.
Enjoy the comic; it's free to distribute and share if you post a link to this page and cite me as the author. Thanks in advance.