first immediate effect would be a pretty spectacular view - Really depends on a lot of factors like angle/trajectory/velocity of the asteroid.

But more than likely the first few hours, nothing much - but after a short time tidal waves, extreme flooding, possibly even earthquakes. Hard to say exactly but it won't be good.

Then comes the shrapnel, after a few months there will be a pretty significant increase in impacts around the world, how large they'll be and for how long - I have no idea. But some will absolutely spiral down towards us.

After that our rotation might speed up, a day could last significantly less time - Seasons may go out of whack.