Well, I prefer to use Norse Swords to make up for the defence of the Huscarls. Theyre pretty good pinning units, and arent terribly expensive.
Mostly I just use FK's to guard my important cities, since (with the defence bonus on walls) theyre damned hard to kill.
I suppose if you had the cash (Sailed down to Antioch, or marched off to Timbuktu, maybe? :P) you could use FK's as the main part of your army, but that 250 upkeep can put a serious dent in your income.