Yep, it is a good move. It's the critical line I believe and is the reason why the 2 knights is considered dubious over 2600. It shouldn't be a good move, but it is.
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It's debatable which move of the following is better, 4.d4 4.d3 or 4.Ng5, Ng5 wins a pawn, but does gives black chances to attack. d4 gives white more attacking chances, but white may have to part with a pawn to keep up the pressure. 4.d3 lets black equalize, and can lead to to a rather balanced position, but it doesn't leave white with any weaknesses.
Well I think 3...Bc5 is considered best. From what I have seen black is fine or more than fine in all variations, although white can make things look quite scary in some of the sharp lines.
It's my understanding that 4. Ng5 is not only a sound move in the 2 Kt defense, but possibly the strongest move and certainly the most forcing. Black, who has several interesting and seemingly adequate, replies, is practically forced to part with some material - for which he has some compensation - but the question seems to be whether that compensation is adequate. Don't be afraid to play 4. Ng4, and don't be afraid to play against it (I almost always play the 2 Kt defense as black if the opportunity arises - with mixed results, of course).
I like 3...Nf6 as opposed to 3...Bc5, but one isn't any better than the other, they often transpose. I guess for black question is which would rather play against the two knights with 4.Ng5 or the evans gambit.
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