this can be an issue but when done well you develop a good rivalry. The hero have a legitimate long time enemy and rival is something that stories lack these days. you have to get the one upmanship down though.
I can agree with that, I liked the way it played out In Death Note, while my best friend thought it dragged on too long.
For Bleach though Aizen left me drainned. Then the latest arc I watched for the anime (not to spoil it too much for people) but that Antagonist was another one who just has an counter to everything it got to the point where I was begging for an end to it.