It really depends on which DLC it is. The Unit packs are nice. None really change the way the game is played but add an amount of flavor to the factions.

In NTW the Peninsular Campaign is worth the money for sure. Overall for NTW the DLC is worth the money.

In Empire TW, I would give the War Path Campaign a miss. It was just not that good. But for the most part all of them are nice to have game additions.