It's a fine method. Do whatever works for you - the casebook itself is presenting you with an excerpted opinion, and, depending on how well it's edited, it might be difficult to really understand what's going on. I know that at least in my case that would drive me crazy sometimes. Wikipedia or a book of canned briefs can steer you in the right direction, but I wouldn't use it as a total substitute (unless you really need sleep - sleep is very important - or it's right before exams). Also, depending on the case it might not be online at all (though most are). And the biggest mistake to make in your first semester is to think that mastering the minutiae of every obscure detail in every line of dicta in every obscure case or in every obscure note is what's going to be on the exam. It won't be. You need to learn how to effectively read and understand a case, to be sure. But what an inefficient way to study (although it's hardly your fault for doing it that way because it's both what everyone else is doing and what you've been told to do by professors, etc.).
I gave up doing formal briefs in the second week of law school and started just annotating the cases in the margins of my book. That works fine for me. Then again, I'm not nearly the workhorse most law students are - I'm neurotic, but a rational neurotic. Unless for some particular reason briefing in the traditional sense provides you a benefit you can't obtain through a less time consuming method, do not just do it because everyone else is. And, in a broader sense, that advice should be applied to pretty much every other aspect of law school as well. And, again, I am not the typical law student, especially at the school I attend. But, ask anyone I study with/talk to and I've got just as good a grasp of the material as anyone else. I just trust myself enough to know what works for me, and to hell with the rest.