I usually watch the OP, if I'm not too excited about how the story turns out, if I am then I'll skip it completely whether I like it or not. The ED, well most of the time I just completely forget about it... lol
Normally if a show is good the developers put a lot of effort into the OP and ED by adding layers of hints in it.
So if i'm watching a show for the first time I always watch them since they can hint towards what is going to happen 3-5 episodes ahead. While the endings seem to support the relationships between characters.
The only time to skip these is when the music is so terrible it actually makes your skin physically crawl.
I almost always watch the opening/ending songs, even when I'm watching 2 or 3 episodes in a row. The only exception to this is if the song just doesn't appeal to me.