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Fully valid and, most importantly, does annoy a user with JS disabled by making an end run around his settings. If the user has JS enabled and your popup doesn't get blocked, it runs the JS and opens the new window and returns false, so it basically cancels the click and the current page stays the same.
If the user has JS disabled, then it doesn't run the code that returns false, treats the click as a normal click and opens the "popup" as a regular page (which is what the user wants - that's why he's disabled JS).
If JS is enabled but the popup gets blocked, it still returns false and nothing happens, though in general, in Firefox at least, the popup will open (user-initiated) and if it doesn't, the user will get a notice asking if he wishes to make an exception for your excellent site.