Good, it's how they excercise. It's what they're suppose to do However flaring continuously would be bad (as would running 24 hrs a day for humans), but they wouldn't anyway, they'd eventually get tired/bored and stop.
flaring is totally normal for a betta to do. Some flare more than others.
Is it good or bad? Well, like shrks1fan said continual flaring may tire the little bugger out.
i would think that it's fine.....i have my room set up so that all of my betta's can see at least 1 other betta. all of them, except the ones on the end of my dresser see 2...they flare when they feel like it....and if they don't want to, then they just don't. i think after a while, they realize that they are never going to get to the fish they're flaring at, so they don't do it as often....but still here and there to try and show off.
most people say that after a while, they just stop flaring and get bored with the other fish.....well, too bad that don't apply to my betta dragon! had him for over 6 months, and he is still vigourisly flaring at his neighbors.
so, no....flaring isn't really bad. if they really wanted to, or if it bothered them so much, then they have the power to look away and stop whenever they want.