Bloggers generally use WordPress because in general, it creates an environment where we can streamline the process of blogging. No need to stress over HTML, fret over styling, or be an expert is CSS, PHP or Javascript – it has all been done for us.
However, this same condition can cause problems, because we will often see the same kind of blog throughout the net – in the form of WordPress templates. WP templates are great because they create a start, but if you choose the wrong one, you’ve suddenly become a face in the crowd.
Personally, I recommend a custom theme, because it means you’ve taken blogging seriously. No non-customized domain is worth its salt on top. There’s the possibility you can continually tweak it over time, which can eventually make something that stands out – but it might have been a better option to simply do that piece yourself.
In general, a decent in the middle is premium WP themes, like those by WPMU Dev. Premium means there’s a paywall to purchase, which isn’t much, but at least it’s something, and in general it means less people use it and also, more thought and work has been put into the customization. However, it’s still an “almost there” type implementation – it may take more work by you to properly do the way it should be done.
The answer generally comes down to where in the process you are, your budget, and how much time you’ve got. Sophisticated businesses almost always start at the end. DIYers almost always go premium or basic. Do what’s right for you. If you’re looking for more advice on how to get started with WordPress (or customize it), I suggest WPMU’s WordPress Tutorials.