how important is a coil

sure i know it sounds like a dumb question, but,

if one coil is giving off a bad spark (6 in total), what will that affect?

1 Coil Will Generaly affect 2 sparks, if it goes to long it will take out the Ingition module, and possibly the other coils

how hard is it to replace the coil? or should it be taken to the shop?

the coil packs (3 of them) are held onto the "cradle" by 2 bolts (If I remember correctly the bolt is 5.5 mm) very simple fix.