[FEN ""] 1.e4 e5 2.Nf3 Nf6 3.Nc3 Nc6 4.g3 Nxe4!?
Some games have followed 5.Nxe4 d5 6.Nc3 d4 7.Nb1 e4 8.Ng1 for example.
At depth 19, Komodo gives an equal evaluation (+0) after 8.Ng1 (and according to Komodo all moves were best leading up to that point after 4...Nxe4). However, it says that 8.Nh4 gives White a +0.32 advantage.
It's hard to judge the theoretical assessment just by an engine's evaluation from the 8th move, but there are only a few games played in my database so there's not much to go on. Overall it seems absolutely playable for Black and funner to play from his side of the board. But objectively White probably has some slight advantage.