Lighted nocks.
Lighted nocks do have their advantages and disadvantages.
Some of the advantages are mentioned in some replies already given.
Added to those, another good thing about using lighted nocks can really guide you with your 'follow through'. Quite often enough, a shooter immediately moves his bow from anchor after the shot to see where the arrow hits...this itself will also change your intended point of aim. The lighted nock allows you to see how the arrow travels and actually will create a subconcious delay to move your bow from anchor. Another 'good' advantage when using a lighted nock.
On the other hand, some lighted nocks weigh as much as 27-28 grains, 15-16 grains more than a standard non-lighted nock, this will change your point of impact a bit if you using both standard nocks and lighted nocks in you quiver. Also the lighted nock will change your FOC a tad bit. It is best to achieve 'more' FOC, compared to 'less'...but here they are things you can do to correct this.