if you do a comparison on energyhelpline dot com you can view details of your current tariff and those of all other suppliers. it depends how much gas & elec you use as to whether its a good price for you. (poss to do the comparison on this site but dunno whether tariff details are available)
bear in mind npower discounts only apply after 12 months continuous supply, but are factored into the final quoted price. i previously found they didnt stay competitive for long! and their service was poor poor poor