Sadly, we decided to drop SilverStripe a few years ago. For years I followed their progress and eventually, once the software was mature, we switched over. We built 20 - 30 sites and started to find the limitations (no system is ever perfect - they all have limitations).
At the time I got to know, through social circles, the CEO of SilverStripe. He'd encouraged them to make the product Open Source which was got them so much attention and accelerated their development. Unfortunately, at that stage, the board was more interested (understandably) in revenue than building open source software and the CEO was asked to leave. Resources shifted to winning contracts rather than 'winning' software which is why they are now the choice for local government.
As a small business, we found the community had dwindled, the software was less reliable, the number of (working) modules for the software declined and the ones remaining weren't maintained (causing security risks). It was sad to drop SilverStripe, but eventually it was the right business choice.
If you are going to choose SilverStripe I recommend approaching SilverStripe.com to build the site or a company that specialises in SilverStripe development. It's important to find a team with a few SilverStripe Developers because good ones are hard to find and there is demand for them.
We choose WordPress if we use a CMS (Content Management System); or Zend Framework as the basis of a custom build (still haven't decided which way to go yet). When building something customised there is a point when the benefits of a CMS are outweighed by a custom build - you can read a comparison CMS vs. Custom Build.
Updated September 2016 - we know also recommend the HubSpot COS as a Content Management System if you want to fully leverage your online marketing.
Feel free to ask questions, happy to elaborate over coffee.