I tend to drink neat most of the time, but I will put some water in some high proofers sometimes (GTS, WLW). Really depends on the day - sometimes I'll taste it neat and if it's not working for me that day, I'll add a little bit of water. Sometimes in the summer I'll use one or two cubes if I'm in the mood for bourbon but not in the mood for the warming effect of it. I don't drink anything on the rocks though, as what I like about bourbon is the smell and the taste. If the alcohol is there without tasting the way I want, I'd rather not have it at all.
I do find that the friends I've introduced to "good" bourbon usually feel like they have to drink it neat, but end up liking it that way. If they've had bottom shelf stuff and had to put it on the rocks or with water, they tend to think that's the only way they'll like it. It just usually ends up being that the quality of the bourbon is the reason for needing ice and not their palate.
The person, their palate that day, the temperature in the room, the temp outside, the bourbon in question, all make a difference. Basically I think it just depends on too many factors to give an easy answer.