Take a look at any Wordpress site and there's a good chance it's using a custom theme, I'm going to take a look at the pros and cons of using them, and my own personal view on the matter.
Using a custom theme can have advantages. If you're a business owner who is struggling to stump up the cash to pay for a fancy custom design and build then purchasing a theme can be a good option.
You can usually find a good quality theme with countless features and widgets for around £50-100 cutting down the cost of a custom build dramatically.
Secondly, you may want to get something online quickly. If you were to go down the custom route it can take time to have the design created to your specifications and the build phase will normally take even longer.
By purchasing a theme you can be up and running in a few hours.
On the flip side there are disadvantages to using a custom theme.
If you're buying one of the more popular themes there's a good chance someone else is too, this isn't ideal as your website will look a lot like many others and in today's world it's good to stand out from the crowd.
Custom themes also come with a bundle of features and widgets which can be overwhelming even for someone with a basic level of technical ability.
This can confuse the user and make creating pages and content more difficult than it needs to be.
Furthermore, all the options provided are useful but the majority of them aren't needed and this can cause your website to be using a lot of unnecessary resources.
My own personal view is that it depends on the individual needs of the client. If you're not keen on spending much of your budget then paying for a custom theme is a perfectly reasonable solution.
However I would personally go down the custom route if you're looking to take your business seriously online. You will get a more unique design for your business and the code will be written in a much more concise fashion which can help with the speed and SEO of your site.
Also using a custom setup will make it much easier to manage the back-end of the site as you can dictate what features are necessary and which aren't.