As Willr said, SilverStripe is targeted more at developers, so knowing PHP is generally a requirement for using SS.
SS strikes me as a step or two above a framework. You're given a framework (in this case, Sapphire), a template system, and a basic architecture with a few pages started for you. What's there can get you up and running quickly if you're just tossing something together quickly for whatever reason and don't really need to do any customization. Where you go from there is pretty much entirely up to you, since you then build from that. Whether this is a good thing depends entirely on your level of experience with PHP and OOP (for you, specifically, it's probably a good thing), as well as the scope of a given project in question.
What I also love, too, is the community. SilverStripe has a smaller community than some of the other big projects, but as you can see, there's a lot of involvement from the people that actually maintain the core. If you post a coherent question on the forums, you're very likely to get a response, often by the people that created the portion in question (for example, if you have a question about the SWF file uploader, you'll likely get a response from its creator, Uncle Cheese). I've even seen devs responding to people that had questions or criticisms in other blogs, which is rare in my experience.