My opinion: Circles, at heart, are just a block. As a block, they can be broken by two things: what they're made of and what they're blocking. They do have one attribute a normal block doesn't - a circle can support a ward. But by itself, it's just a block.
This is a bit different from how I treat circles. Basically I treat a Circle as a Threshold, since from my understanding of the RAW and canon, a supernatural threat or caster can beat on a Circle as much as they like but still not be able to breach it in a way which would cause it to collapse like a Block can. Additionally, a 'vanilla' mortal can make a circle, all that is needed is something to 'draw' the circle with, a drop of blood, free will and intent. If enough mojo is put into something, part of it can make it through a circle, but the circle will soak up and suppress at least some of what is being sent across.
Of course a circle can also be easily broken by a mortal with free will and intent who steps or sends something physical over the circle. Woe be unto the mortal who feels safe from my wizard behind a circle... I'll just shoot them with my Mossy loaded with Dragons Breath rounds. Given that most circles are fairly small, one of a few things will occur.
Either I'll be able to tag a temp Aspect: "Inside a Circle" for a bonus on my attack roll and be able to take down the circle, shoot my opponent and using Dragons Breath rounds (Google them they're fun...) give my opponent the Aspect: "On Fire" while inflicting stress on them, or they'll be forced to step out of the circle to avoid getting shot, or I'll at least drop the circle. Now if they'd decided to draw a very large circle, I might not be able to hit them, but I'll at least be able to drop the circle.
Now, one area where I'm not quite sure I'm happy with the RAW when compared to canon is the strengths of Thresholds, which impacts on how I handle Circles.
Harry, when he crossed a Threshold in an apartment building in White Night without an invitation, seemed to leave something like 99% of his power behind, leaving him with only enough magic to cause his pentacle to glow and provide light. Given that Harry by now was somewhere in the top 30 - 40 wizards in raw strength, that sort of makes me think that even a fairly weak home Threshold should be starting at Legendary. Admittedly, that is just my opinion and something I'll be testing and refining during a campaign.
-Cheers
