Yes, I believe that ISIS carried out the Charlie Hebdo attack in Paris, because they wanted to punish the magazine for making fun of Islam. They said that they were avenging Muhammed for the cartoons. They are the only people with a motive to want to inflict this kind of harm for free speech.
It is and will be hard to pinpoint ISIS to the attack, as any number of terror groups could be behind this massacre. However, given the current state, it is likely that the terrorists will use ISIS as the excuse for the attack. Regardless, a vicious and sadistic terror group is behind the deaths of innocent civilians. It matters not who did it.
In the case of many high-profile terror groups such as ISIS, the group seldom carries out any type of attack without explicitly taking credit for it, precisely because they relish this sort of publicity. We haven't yet heard of any group taking credit for this attack, and this leads me to believe it must not be ISIS.
I am not at all knowledgable about ISIS. Only they would know if they are responsible. Anything else is just speculation and guessing. People are innocent until proven guilty. It could be a copycat group or someone else altogether. It could be college students, who knows. Can't say it is ISIS.