I have seen Imperial Knights in some form (errant, paladin, etc) in pretty much EVERY game I have played in my local meta. One guy in particular has a hard on for them, bringing no less than 2-3 of them. He must have robbed a bank as he has bought something like 6 of them in 2 months!. I expect to see more and more of them in the future.
I have one of two ways to deal with them. Firstly, kill teams in drop pods (preferably sternguard with combi meltas and tactical doctrine, if running ultramarines). May be an expensive commando unit to take it out, but if a squad of ten sternguard manage to hit it with 7 shots (average), then there is a possibility with the melta rule that it will suffer at least 4 saves on its ion shield, failing an average of 2-3 of them.
Now, support these with T/L lascannons or missile launchers/lascannon devastator squads and you (might) kill it before it can get in and ruin your day.
The toher option, spam TH/SS assault terminators at it. Assuming it is the batle cannon variant, it will need to get into CC to really hurt you. 2-3 waves of these, assuming they are protected long enough to get into combat, will do the trick most of the time (unless you are unlucky on your save rolls and D-strength wound allocation)
Im not saying this works all the time, but it has saved my bacon a number of times.
In all fairness if you can get a knight in your list then do so, but it just means that your list will be tailored to kill this superheavy, something which people seem to frown upon during some games. IMO if you have a knight, be prepared to face a list tailored to kill it, and dont get upset if it is nailed first turn