(How) should I take notes on a subject for self-study? Suppose I am interested in really learning / thoroughly reviewing some subject (e.g. the basic theorems of infinite Galois theory, or the classification of compact Lie groups).  One approach I might consider would be to write up a set of notes for myself going through all the main definitions, notation, and proofs of the main theorems in the subject, with appropriate references.  I can think of several good reasons to do this:


*

*I can't find a single textbook that does everything in quite the order I want it done in, or proves things quite the way I want them proven, and I want to synthesize material from multiple sources.

*Just the act of writing down all the proofs in my own words will force me to understand the flow of ideas better.

*I can revise the notes as my understanding of the subject improves.

*I can put the notes online for others to benefit from.


But suppose one day I find a textbook that does things nearly perfectly (e.g. Neukirch for the basic theorems of algebraic number theory), and I am having trouble seeing the point of "competing," as it were.  Is it still worthwhile to write up the notes that I was planning?  Should I just do exercises instead?  
(Feel free to interpret this question more generally, e.g. to ignore the body and concentrate on the title.  CW because this question is mildly subjective and I'd like to encourage a broad range of answers.)
 A: For me taking notes instantly on a subject for self-study is very, very important. Here I can check proof details, ponder about some relationships with already known topics and get intuition for the theorems etc. and how they interplay. The latter is rather difficult to communicate - therefore the author often does not put an effort into this - whereas the former are just personal issues. This makes these notes rather individual. Everybody has to make his own notes to get new insights from them, right? After having finished the notes, I have internalized them and just don't chuck them because usually some day I realize that they are useful without remembering the details.
Now I must admit that your question goes into another direction, namely you want to make a well-written summary of a theory and put it online. I used to do that very often some years ago (on www.matheplanet.com), but nowadays I think this is only really useful when you know that you are not reinventing the wheel again or when you enrich the theory with some very interesting interpretations which you have never heard of and about which you want to discuss with others. Otherwise it will be "just" an enrichment for the readers who do not have to go through a whole book on the subject or have found your summary instead of one of the thousand other ones online. I think when you put a focus on your own learning success, you should probably do not waste your time with such a summary.
A: I have to agree with Martin on the virtue of drawing up personal sets of notes. But sometimes it's hard to keep that going on your own, because there are always so many distractions, so many things to learn and do. 
Actually, one of the most effective ways of thoroughly learning a subject is by teaching it. :-) Even if you are an undergraduate, this can be done in the context of a student seminar: find some fellow students at about the same level as you are and organize a seminar on a topic of mutual interest. This will force you to master many details of a subject, and the joint effort helps to keep each individual's energy from flagging. 
Or, if you can't teach a class or schedule a student seminar, and your topic doesn't feel quite right to blog about, consider contributing your growing knowledge to a wiki like the nLab. There is plenty of scope to offer your own personal perspective in such a venture, and you will likely get plenty of feedback. 
A: Just an idea for the "How" part.
You can try with a local wiki. For example, Instiki is pretty easy to set up and it's the one used in nLab where the purpose is "to provide a public place where people can make notes about [category-theory related] stuff".
