it can be really bad like the car feels like a bag of crap and sounds like a rally car cos its running on 3 cylinders
or it can be less obvious like just a cylinder misfiring under load so every now and again when u boot it you lose power
depending on what engine u have the check light may come on or in my case it flashed at me when the misfire occured but then went out after
i bought new coilpacks recently from euro car parts for 14quid plus vat, expect to pay around double at a
dealership