LOL.
I agree with going f/2.8. I had the f/4 for a while, loved that it was light and very sharp but I ended up needed the faster glass. I ended up splurging and getting the f/2.8 with IS. I haven't been able to compare IS to non IS with the same f/ stop but I definitely thing going with the fastest glass possible is helpful, especially in sports.
I noticed a big difference when taking pictures of my dog outside chasing the frisbee. IS won't let you "stop" the action like a high shutter speed will. I do find the IS to be super helpful when I'm taking portraits indoors though.
To me the only drawbacks to IS are the price and the extra power consumed. I'm not sure if it adds any weight or not, but I definitely feel the weight difference between f/2.8 and f/4. Well worth it IMO.