Im like fed it depends on the situations. Ive brought on players before to bolster a narrow lead, and theyve gone on to score, Ive brought on fresh attackers to try and snatch a draw or win and theyve made little or no impact at all.
I think its only an option to use when neccesary, ie when a players fitness level drops or if you are winning and want to preserve a player for the next game. | 
