I don't have that much experience in the field, but here's what I think:
Numerically, the vast majority of people who try domaining lose money.
(Naturally, just like with anything else, those who really make a commitment to learn and stick to it have a much better chance)
The average domain sale price is much, much smaller (both mean and median, but particularly median=most common $ amount paid) than most dreamers looking to enter the field think.
The business is very "incestuous" in the sense that most domain sales are from one domainer to another
(Yes, a sale is still a sale, but enduser sales usually bring more $).
Study a lot, understand what makes a domain valuabele, what makes one worthless, how to sell domains, etc. etc. before you start buying, otherwise you are throwing away money.
Like in the goldrush, the majority of those who sold the showels made money (registrars) and the vast majority of those who were looking for gold (domainers) lost money or at the very least made much less money than they would have by just working a "regular job". (Yep, sure they did have more adventure and flexibility and a slim change to strike it big etc.)