Some important things to realize about Circles, Sparks, and the overall philosophy of Google+.
When posting, Circles are for restricting access to posts you make. They're a privacy control. Circles only control who cannot see what you're posting, they lock people out. Circles are not a good way to post certain content to people who are interested, because you have no control over, and no idea, how they have you circled. They may have you circled for your political posts even though you only have them in a board game circle, in which case they're not interested in your post anyway and you're wasting your time restricting access to only them.
When reading, Circles are for defining your relationship to people. Circles are not related to content of posts, if you put someone in a circle called Politics they can still post on any topic they like. You can add a science professor to a Science circle but it he may only talk about rock climbing, and then you'll see rock climbing posts in your Science circle. Instead, circles are relationships. For instance, if you want to see posts only from your family, use a Family circle. If you want to read posts about a particular topic, that's what searching and Sparks are for.
Some people primarily post about certain topics, like technology bloggers for example, so you may add them to a circle to see what they are going to say without having to search for it. You would do that if you find the majority of their posts interesting (in other words, they're an interesting person to you). If they are in a circle you will see their posts whether or not you are interested in the topic of a particular post.
To summarize all of that, Circles define relationships, they do not relate to content. It usually doesn't make sense to add someone to a circle that only occasionally posts on a topic you are interested in.
Sparks are for finding specific content. You enter some keywords like a typical Google search and G+ returns posts that match that search. It returns people in your circles first, so you will see posts from people you care more about, but it finds all public posts. Also, note that all of your circles have the same priority in the results, people in your Food Critics circle appear at the top with Family, if their post happens to match your search. Google+ has no way that I know of to read only certain content from only a certain group of people, I don't think that's what it's designed for. Sparks have other limitations, for one they only search for keywords but the Google+ community doesn't use Twitter-like hash tags, so you don't find all of the relevant posts you could find. Maybe Google will eventually let you set filters on specific circles, but as of today this is how it works. It may be that tags will become popular instead, which seems more likely to me.
Based on all of this you can conclude that Google+ primarily encourages open and public conversations, the privacy controls are effective but mixing content filters with privacy is not. I think some people are having a hard time getting used to the concept of the public conversation, particularly coming from sites like Facebook, which hasn't had a public forum at all (until recently, in response to G+, but users there understand it even less than here).
Like many people I've added lots and lots of people to circles lately, due to circle sharing becoming available, and I've seen my stream become busy and confused as a consequence. It doesn't make a lot of sense to have so many people in circles, it makes it harder to find the content I actually do care about. I've even had people in these circles request that I only post to them things that are interesting for them, which is simply not how Google+ works. I'm hoping by sharing a little of what I've learned that maybe I can help people understand a little better how to use Google+ effectively.