mwh: are you referring to Goodwin's Greek Grammar or his Greek Moods and Tenses? Despite its years, GMT is a very useful resource. I'm not familiar with the grammar, having used Smyth.
What parts of K-G are most useful for non-German speakers?
I think it would be very difficult to navigate K-G without German. Some of the grammatical terms are similar to English, but some are not. The main advantage of K-G over Smyth is the wealth of examples. mwh says that Smyth plagarized K-G, but it's useful for us English-speakers, even those who can .
Author-specific grammars (mostly treatment of syntax) are mostly digested in commentaries, where specific passages are discussed. Unless you're writing a commentary or editing a text or working in depth at a very advanced level of scholarship, you would probably find the materials in commentaries adequate. They're generally very expensive--you need access to a reference library. Smyth is mostly Attic, with nods in the direction of other dialects (though apart from koine and Homer and Pindar, most everything else survives only in fragments anyway. I think that the "Doric" of tragic choruses and the "Ionic" of tragic dialogue is mostly lightly Doricized or Ionicized Attic.) There's a lot of grammatical material specific to the New Testament, though I'm not familiar with it, and in any event koine is not so far removed from Attic.
The one exception is Chantraine's Homeric Grammar, in French, which is indispensable at a certain even moderately advanced level and is merely outrageously, not astronomically, expensive. It's not really well organized for those at a beginning level, though. LIttle of it is original to Chantraine (as he admitted) but it collects all the work on Homeric morphology (vol. 1) and syntax (vol. 2) through the mid-20th century. Most anything after that you would have to hunt for scattered in articles in learned journals, I think.
I hope that mwh can stay at least occasionally involved despite other commitments. I personally feel much more comfortable giving suggestions knowing that a grown-up is looking over my shoulder and will jump in whenever I disseminate misinformation.