In my religion...
Method javadocs are necessary to document method contracts. For all my code, no method returns null and no parameter can be null unless specified in the javadocs. Eg "@param moo Can be null. @return May be null." I also use javadocs to let the user know about a default value. Eg "Default is X.". Javadocs are also needed to document what methods can be overridden by a subclass and in that case, what the default implementation does and if super must be called. Method javadocs are NOT the place for stories about how everything works, example code, unimportant implementation details, reasons why alternate implementations were not used, etc.
Class javadocs explain what the class is for and can be used to also explain how it works, but should be concise. External documentation is the place to go into great detail. Some people try to outline the entire usage of a class and end up referencing many methods in the class javadoc, but I prefer explanations on methods where it makes sense.
Implementation details do no belong in javadocs at all unless it is somehow relevant to the user (eg, big O). Non-javadoc comments often contain implementation details and are necessary when they help code to be understood. Think of them as "private" comments. I use them extremely sparingly and as others have mentioned, often code can be written so that non-javadoc comments are not needed. However, javadoc comments are different and are needed to clarify method contracts so that people know how to use your code.
Remember that comments do not replace real documentation. As such, keep them concise and simple, incomplete sentences are fine. In ALL my comments and documentation I try to avoid the word "you". Give it a try, it makes everything more professional and more concise.
Of course, all this is only relevant when writing OSS libraries or when coding for work where others may need to maintain your code. For private projects, do whatever the hell you want.
I use spaces, because if I copy and paste code into a bug or example, tabs get reformatted to something weird, or sometimes just stripped outright. This is made worse when tabs and spaces start getting mixed, which is always going to happen when you want to align things like long parameter lists, because they never manage to line up nicely on a tab stop. I'd use tabs everywhere if the formatting weren't so damn "unportable".
So don't align things. No ASCII art in source code! I use Eclipse to format, so never have mixed tabs and spaces. Tabs are better because the individual can decide how wide they want a tab. I prefer a 3 column tab, as 2 columns isn't quite enough, and 4 is a bit too much and 3 gives me horizontal space.
