From what I can recall, the Comets that arrived in time for the offensive through the Ardennes did not see any (or very little) action. The crews were not properly trained to use the new vehicles and were reverted back to using their old Shermans/Cromwells.
At least this is what I can remember, but I may be getting confused with something else.
Yeah the plan was for the 11th Armoured Div to re-equip with Comets in December '44. Hitler, clearly fearful of this new British tank, ordered a huge offensive to hamper the process.
11th Armoured finally got their Comets in March '45, and the division went into combat with it's new mounts on March 28th.