For sieges I use their fire arrows and park them with my other missile troops. In open battles I tend to just stick them in melee mode and use them like light cavalry going for the missile troops. If the AI army has reinforcements then I find that they try and meet up the two armies and pretty much ignore my troops until then. In this case I'll use them as missile skirmishers and harass them as they move across the map. I always put my general in a heroic cavalry unit, they are way better. Now that I can recruit veteran and heroic rider units I've stopped recruiting chariots altogether, but the ones that I have had from the start are actually pretty badass at this stage.
Cheers mate will have to give them another go with that in mind, I do always tend to favour cavalry but quite fancied giving chariots a try, then by the sounds of it used them entirely wrong!