I started getting mine waxed at 15, though I switched to threading a few years ago because it is better for my sensitive skin.
No disasters here. Frankly my natural brows were such a disaster that just about anything else would have been an improvement. I had a unibrow and they were bushy and long. The first time I got them done I remember the woman being astonished at how far they extended...just about merging into my hairline. There was just no way I could have done them on my own. They were out of control. It's only $12 a month for me to get my brows and lip done, so I'd rather leave it to the pros.
Like Steph, I don't go too thin. Pencil thin brows look weird, and it's a lot easier to take a little off than grow it out. I do, however, get the tops done...just because I have to. You may not.
I guess the moral of the story is: If you aren't a hairy beast like me, you can probably get away with doing it yourself. I'd get it done professionally once just so you have an idea of how they should look, and then the maintenance will be easy.