I love being able to get all the girls in the end but even more so I hate having to pick one girl to end it all with, so I always pick the harem ending.
Depends if I like the girls. I have no problems with harems even if I don't particularly like some of the girls, but if I don't like one (or more) I don't do harems. But I don't know if people consider having like 5 girls (out of 10, for example) a harem, or if they all have to be in it.
I always go for the harem ending, especially on the first run through. Maybe if I read through a walk through later and realize I missed out on some really tasty scenes I'll play back through with one interest to catch those scenes.