Carriers are very strong if used correctly. First of all, they are not really good vs zerg because scourges combined with hydras will own them in most cases. Carriers are best in PvT but they are also good in PvP.
Second, carriers are very expensive. There is no such thing as a working carrier rush. If u are going to make carriers u need several expansions and u need to hide them from the enemy so that the initial attack will catch him off guard.
Carriers are great expansion killers, and are very hard to beat if cliffed. Allways remember to have 3-4 observers following your carriers if the enemy is terran, cloaked wraiths can kill off carriers very fast. Also, combine them with masses of zealots. A well played carrier-lot combo is the hardest thing a terran can face in late game on lost temple.