Canon EF 70-200 mm f/4L IS USM
I have upgraded from the non-IS 70-200mm f/4.0 L to this amazing zoom lens.

Sharpness	I agree with most review sites that the EF 70-200 mm f/4L IS USM is Canon’s sharpest zoom lens. It is definitely sharper than my old non-IS version of the 70-200 mm f/4L. Although I did not do any direct comparison, I think it is sharper than my EF-s 17-55mm f/2.8 IS. See how sharp this f4.5 100% center crop 50D shot is. On a 1.6x crop body, the corners are almost as sharp as the center.
Chromatic Aberration (CA)	Even when you pixel-peep on 50D shots, CA is still almost invisible.
Image Stabilization (IS)	This is the best part of the upgrade. It expands my photographic opportunity to twilight cityscape, stage performance and indoor candid. The spec says 4-stop improvement. This handheld shot of 200mm at 1/13s comes pretty close.
Auto-Focus	For low light situations, focus is fast, accurate and silence. When mated to a 5D MkIII, AF tracking is excellent.
Close Focus	With a minimum focus distance (MFD) of 1.2m, it provides .21x magnification at the max zoom of 200mm, better than both versions of the f2.8. With IS, it can take handheld close up photos of flowers and butterflies with good working distance.
Physical	About the same size, weight, and high build quality as the non-IS version. At half the weight of the f2.8 versions, I rather carry the f4 around for a day. Its white color does attract attentions. It also gets respects. In crowds, people tend to make room for me.
Price	Only you can decide whether it is worth almost twice the price of the non-IS 70-200mm f/4.0 L or the non-L EF 70-300mm f/4-5.6 IS to get the best of both and more. Canon charge an outrageous $140 for a tripod ring. But with IS, who needs tripod?