I never "knee-jerked" on a GW product before ever, not once in nearly twenty years, until I saw the Imperial Knight. I bought three!!!
As far as converting, there is a good little tutorial for bending the left knee, in the PCA forum. It looks very easy to achieve. I personally left all of mine intact and just went with rotation of joints to get variation. It really is a fantastic kit to build and paint, I thoroughly enjoyed every minute, something I haven't felt in quite a while with the modelling side of the hobby. So much so that I was a bit gutted when I finished the third one and almost immediately bought another!!!
It is also a very big area of base not to do a bit of work on too. I elected for the GW 40K basing kit (obviously other options or suppliers are available) and used the left over sprues for bits of rubble. I didn't go overboard as I wanted the focal point to remain as the model with the base an accent to it.
Playing wise I can't really answer your question as you can see that with three I'm down the route of allying other things into my Knights rather than the other way around. However, one thing I can say with certainty is that if you have one in your army and so does your opponent avoid close combat like the plague. It will end badly for one of you, if not both. Imperial Knights swinging Reaper Chainswords looks amazing in the minds eye but ends very badly on the table, and very quickly, like one round of combat quickly.