I've only tried a few PC-Engine games on Wii and that with a GC-controller so I wonder: how is the feeling playing the old mario games on VC? Is there any lag on the controllers? Is the timing any different?

As far as I can tell, the timing is the same as it always was... No lag at all!

I love it. I would highly recommend that you use the classic controller though. Many vc games from all systems support the GC controller but due to the button layout many games are uncomfortable. I first realized this when I played Super Mario World as an example.

Now, I sold my wavebirds and have two Classic controllers. I only play NES and SNES, but it is perfect for all the SNES games. NES is obviously just played with the Wiimote.

personally I use the Classic Control for the 8bit and 16bit games.

And I use the GC control for the N64 games ;)