by Eeli Kaikkonen » March 27th, 2012, 6:14 pm
I can speak from experience only about vol. 3; I own it. It's technical and a bit old English (you could say "scholarly"), not easy to read. I read it years ago, when I knew Greek even less than now, and I didn't understand much of it. Compared to Robertson there's probably not so much difference in writing style or difficulty that it would affect your decision.
The real issue is Turner's view about Semitic influence. As far as I know Moulton originally wanted to show that NT Greek is ordinary Koine. Turner, who continued his work, turned it upside down (pun intended). Robertson held the same view as Moulton and did what Moulton didn't have opportunity to do - showed in his grammar how NT Greek is just Koine. (See Robertson 3rd ed., Part I Ch. 4). I don't think this actually affects Turner's categories, definitions etc. but if you want to get a realistic picture of NT Greek within Koine he's not very helpful. When he handles example passages he routinely appeals to Semitic influence. Looking synchronically and diachronically he's alone in his view and isn't recommended for that. This doesn't mean he isn't helpful at all, of course.
As much as I have read opinions of others, vol. 1 is often recommended, vol. 4 is antirecommended. Vol 3 is used often as a reference, vol. 2 is not. I haven't seen vol. 2 and can't say how much it helps in understanding Koine, but I'm inclined to think that it gives more technical and historical than practical knowledge (even compared to other technical grammars). I remember seeing only one reference to vol. 2; if I remember correctly it was in M. Erickson's Systematic Theology and was probably a linguistic fallacy (seeing a certain kind of action in certain kind of accidence, which is similar to history-of-word-usage-tells-what-it-means fallacy.)
BTW, at least vol. 1 can be easily found in electronic format.