From my experience lag is fairly low and often not very noticeable. It is much better than my experience with Kaillera. Of course, the huge advantage of the netplay plugin has over Kaillera is that it works with Project64 2.3, the best N64 emulator in my opinion. You can use the netplay plugin with other N64 emulators, too.
I recommend going to Options, Settings, and unchecking “Pause emulation when windows is not active?”. That way you can adjust the lag easily while the game is running. Otherwise, every time you click the window that lets you adjust the lag setting, the game will pause.
Let me know if you encounter any bugs.
Enjoy Project64 2.3 with netplay support!