All I can say on this subject is this, its like buying any third party economy product. Its not going to be the equal of an OEM Nikon ED glass lens or a Sigma APO but it also as you said did not cost the same. Last week while I was looking on the APUG for sale ads, I came across a Tamron 70-300 f/4-5.6 AF macro zoom for $40 plus shipping, the seller said there was nothing wrong with the lens other than it had some internal dust specks which did not affect IQ what so ever. The lens arrived on Thursday, I spent the last two days playing with it, its an older lens designed for the screw drive on on a D200 or an F100 etc etc etc.
Which means it will AF on my Nikon N70 but I have to manually focus it on my D40 , it meters on my D40 just fine in Aperture priority mode and it works in every mode on my N70 so I am tickled. Further more I cant find any of the internal dust he was talking about. So I have no complaints from any of those stand points. The only gripe I have is it is soft, not horrible but it is not as sharp as my 55-200mm Nikkor by a long stretch either. It will do what my 55-200 Nikkor will not do though and that is focus down close from 180 to 300 mm and its not horrible in macro mode, not great, just not as sharp as a micro Nikkor or a Sigma macro.
However I paid a total of $50 thats it. Money well spent I did not have the cash for a $200 to $500 macro lens. Plus in non macro mode its not bad and I will use it a lot not just sometimes. It will get used anytime I need a 300mm lens. Beggars cant be choosers , the way I see it you are getting an awesome deal, you paid $0 you get some long glass you did not have and you get another tool in the bag. Its a no lose situation, use it and enjoy it learn with it. When you get the money for a better lens pay it forward and give your free lens your friend gifted you to someone who cant afford a nicer lens who would like to have it who needs it so they can learn and take photos they would not be able to without it.
I think it's an awesome gift your friend gave you and you are in for a lot of good fun with it enjoy
