After 4.Ng5 d5, the main line, black often stays a pawn down for a long time, having active pieces as compensation. This is attractive to some people, while others find long term material deficit off putting. Objectively it's totally fine for black.
White can also play 4.d3 though, which will force the game into some other variation like the guioco piano or hungarian (i.e. the same as if black had chosen 3...Bc5 or 3...Be7). So even if you plan to play 3...Nf6 you'll have to learn something about 3...Bc5 or 3...Be7 positions.
I just followed a game between Tal-Miller in 1988 and Miller was beaten with this defense. It was quite an exceptional game. I hardly use it mainly using the Philodor against the Italian game. I know that White likes to bring the Bishop to c4 and the Knight to g5. Any thoughts?