IS or not?
Nowadays Canon produces tele-zoom or tele-prime lens with IS (IS stands for Image Stabilization), such as EF 70-200mm f/4L IS USM, EF 70-200mm f/2.8L IS USM, EF 300mm f/4L IS USM, EF 300mm f/2.8L IS USM, etc. Lens with IS has advantage for taking a picture in low light condition without tripod, IS also can reduce blur because handshake. Handshake is a user problem, each user has different problem (for example someone still can take a photo with shutter speed 1/40s, but others can’t). Remember that IS only use to reduce blur because handshake problem, it can’t help reduce blur if the object moves. IS only help you when you forget bring tripod; if you bring tripod all the time, it’s not necessary to have lens with IS. If you think that tripod sometimes becomes a “burden” maybe you need lens with IS. IS also can reduce noise; if your lens has IS you can reduce ISO, that’s mean you can reduce noise. I have tried EF 24-105mm f/4L IS USM (you can see my review here), IS is useful when you take a picture, especially when I used focal length 105mm. Lens with IS is more expensive, for example you can compare EF 70-200mm f/2.8L USM with EF 70-200mm f/2.8L IS USM. The difference in price is much, you can buy another lens for it, so my suggestion if money isn’t your problem – get the IS lens instead of non IS lens; if money is the problem you should think it twice “do I really need IS” because I still can bring my tripod. You can see the detail about Image Stabilization here.