Definitely an amazing lens. I've had the opportunity to do some shooting with it, and it certainly is worth the money.... for certain applications. For instance - indoor sporting events. Take that to a basketball or hockey game, or gymnastics, wrestling, etc - you'll have a distinct advantage over slower lenses (like mine).
I don't recommend using the teleconverter for indoor sports, however, as its going to drop your lens a stop or so. Just sit as close as you possibly can. (Even professional photographers with their 300mm lenses sit courtside at basketball games.)
However- if you want to shoot outdoor sports or wildlife- the teleconverter is probably going to come in very handy - especially if the wildlife is skittish and you can't get very close without disturbing the creature.
I'd add that for future purchases, you might choose a certain type of photography first, and build your equipment around it. After all- you wouldn't buy a souped up desktop computer that is aimed at high-end graphics, only to find that you don't use it for gaming or video editing, and like to just use it for word processing, and that you'd like to travel with the computer. In my experience, its better to buy cheap equipment and lenses to get started, try out a few different things to see what you like to do with it, and then sell it and buy nice equipment that is designed for your purpose.
The nice thing about your lens choice, is that there are few things you can't do with that lens.... buildings, panoramas, macro, and confined spaces come to mind. But you're all set for sports, wildlife, portrait, still-life, etc.
One cheap, versitile canon lens that I think almost everyone should have- the 50mm f/1.8. Its only $80, and is a prime lens (no zoom), but at f/1.8- you can easily blur out your background and/or shoot in extremely low light. I've even used this lens to shoot some indoor gymnastics at close range. It takes great looking shots. Those are my 2 cents. Enjoy the lens! I'm very jealous!
