I personally don't have a problem with ponytails. Like Bliss also said, some people are able to pull it off. As far as being taken seriously goes, I agree with you there. If you're an artist (musician, painter - whatever) that's fine. Most artists tend to grow out their hair anyways. But if you're a serious professional, say like a doctor with a ponytail, I don't think I'd come to you for a check up.
It is very, very rare I see a guy that I actually think looks good with long hair, but when I do it is almost always no longer than shoulder length. I understand the reasons for having your hair pulled back in a ponytail, but I don't think I've ever seen a guy that I found a attractive with it pulled back in a ponytail, but that just isn't my thing. Other than that, I really don't have a problem with it. To each his own. =)