Ok, photography geek here, so this may be more than you care to know.

One thing to remember about this (or any big zoom lens) is that it's gonna be relatively heavy. This one is about 1.2 pounds, which doesn't sound like much until you lug it around your neck all afternoon.

The other thing is that, the lens you mention above is f/1.8. That's way wide open. On the zoom lens, when you zoom into 300mm, you'll be maximum open at f/5.6 (the f/4-5.6 indicates the maximum open aperture at the zoom extremes). That's not uncommon for affordable zoom lenses, BTW, as the mechanics inside to keep that constant across the zoom are very complex. The problem is that, the zoom lens is going to require you to shoot fast (shorter shutter times, higher ISOs) to avoid camera shake if you're zoomed all the way in and not on a tripod. So, just be aware there are shots you can get a good exposure on in the 50mm f/1.8 that, under identical lighting conditions, you'll be unable to get a good exposure on the zoom.

Thankfully, all of this is lessened since you're probably talking about doing a lot of shooting in bright sunlight (at soccer games). I imagine you'll be able to crank up the ISO and have it all turn out just fine.

If you're at all interested in learning more about photography, I can't recommend the book Understanding Exposure enough. It's a really good introduction to how to get your camera to take pictures such that the light, focus (depth of field) and motion (blurred or frozen) are just what you want, and it's not hard to get started with.

I have that very same lens and it is by far my most favorite thing in this universe. I use it for the kids sports, but it is a little too zoom for bball. Buy it...Live it...Love it.

I have the Canon 75-300mm lens...There's a large part of me that wishes that I would have waited to buy that lens with image stabilization. Yes, I own a tripod, but I typically don't like having to set up on a tripod, I like to just take pictures.

This book is totally worth it too: http://www.amazon.com/gp/product/1598633376/ref=wl_it_dp?ie=UTF8&coliid=I2YF69V7OWLSZK&colid=2T4GUB8VDNAHB

this discussion proves to me that what I really need to do is find a class about photography for dummies - I accidentally turned the delay on a few weeks ago and had to screw with it for hours before I figured out wth i did wrong.

If you go to the tamron lens site, there's a place where you can try out the lenses on the computer, like click on the lens and a picture comes up and arrows that let you see how far in and out it will zoom.

It helped me figure out how much I needed the numbers to go up and down.

I'm loving the 50mm also and bought it with the body, so I'm needing something with a wider angle because I have to keep backing up to get all my boys in the picture.

And I just spit out ALL my photo knowledge right here in this comment.

Thank dog for facebook and blogfriends on facebook so I could see this post and know that there might be fabulous comments going on. WOW.

Glad you asked this marvelous question, BusyMom. You are a goddess...

I'm also a photo novice but can report this: as Brett mentions above, it is lots bigger than the 18-55 you have. I have one that zooms up to 270 and I love it, but I only use it for particular situations. An afternoon at the soccer field might be such a situation. :)

I have the Canon Dig. Rebel and the EF 75-300mm zoom you speak of; I love it!
It isn't as big and bulky as those big white and black zooms, but still large enough to capture what one would need, say, at a ball game or in the mountains.
It is obviously going to be bigger than a traditional lens, but I don't find it to be so big that I don't bring it along. The key is a good camera bag/protection device that isn't huge and will protect your stuff.
The "Zing" Camera case is a really good one for this, although it does look a little naughty.
Good luck.

I'm coveting the 70-300mm too. I have the Rebel XTi...I love my 18-55mm...great in a lot of diff. settings. Have you checked out www.thepioneerwoman.com? She has great advice about camera equip. and she has great shots.

Good luck!

I have a Rebel XT too (with the kit lens and the Canon EF 50mm f1.4 USM) , and I have the Sigma lens you're asking about. It's OK, but I wish I'd held out, saved up a little dough, and gotten a Canon vs. the Sigma. It is heavy (Canon equivalent might be too) and not as sharp as my Canons. It's a lot more prone to lens shake than my two Canons - you definitely have to use a tripod, and the light needs to be pretty good.
It's an OK lens, especially given the price, but I use the kit lens more than anything!

I wish I could be of help here, but this is EXACTLY why I went with a SuperZoom Camera over and DSLR. (THough I did by the Telephoto expansion Lens recently).

I've been looking into the Sigma 70-300 lenses myself (I only need it for web resolution pictures, not fine art, and can't justify the price of a Canon lens right now). The overwhelming consensus seems to be that it's worth the extra $80 to get the one with APO.

After shooting the Lake Michigan Polar Plunge today with an entirely inadequate 50mm lens, I think I'm ordering this bad boy tonight...