I'm another mum to a boy, and a big fan, but I have to confess I wasnt when I first found out. I think it was because I really thought it would be a girl, could only think of girls names, the only baby I knew was my niece and I had that "mini-me" fantasy that someone mentioned earlier. However, I am very glad that I found out while pregnant, and quite early too (after amniocentisis at 16 weeks) It was really great to have time to adjust to the idea, by the time baby arrived I was totally into the idea and fell in love with him the instant I had him in my arms. I feel sorry for the lady who said it took until her baby was 4 months old to really get used to the idea, or anyone who felt disappointed at the birth. I am glad I felt my disappointment early, got over it and was thrilled and overjoyed to welcome him into the world on his birthday.
I also think I have enjoyed being a mum to a boy much more than I ever would have expected, precisely because I had no expectations or fantasies about what it would be like. Instead I have got to know the person who arrived in the world, rather than having any preconcieved ideas about how it all "should" be.
So my view is, boys are fab, disappointments dont last long and are nothing to feel guilty about, motherhood is a whole whirlwind of emotions and there is no "right" or "wrong" way to feel about any of it. Boy or Girl, you will love the experience 
oh... and I am having another boy and am very happy about it!