Overall the 300/4L IS is an incredibly versatile lens with excellent image quality in this price range ($1000). I recently swapped the 400mm f/5.6L for this 300mm because I need something in the 200-300mm range for candids with my 5D, I rarely need 400mm, I want something that can get much tighter head shots than my 85L, and the extra stop and IS come in handy in a lot of low-light situations such as stage performances. The 4.9 ft. minimum focusing distance (MFD) allows for some killer close-ups (even macro shots) - it's like having a completely different lens from the one originally intended (distant candids and sports). I'm extremely impressed with its sharpness for close-ups. The IS is really key for low-light shots of stationary subjects, but you need to keep the shutter half-pressed for a bit to allow it to spool up. The bokeh at f/4 is much better than expected. The images look sharper, more contrasty, and more saturated than those from my previously-owned 200/2.8L II or 135/2L, and that's saying a lot because those are two of Canon's very finest. I've used the lens at f/4, but I see a noticeable sharpness improvement at f/4.5 so use it there usually. I don't see much loss in sharpness with the TC 1.4x (420mm).
Focus reliability with moving subjects is a little more hit-or-miss than I would like. It's not an extremely fast-focusing lens like the 300 2.8L IS or even the 200 2.8L II. I sent the lens in to Canon for AF calibration and it did come back noticeably sharper in general, though the not-so-fast focus speed still affects reliability with moving subjects. Turning off IS helps with randomly moving subjects as long as the shutter speed is fast enough. The 5D is not as good as the 1DIII at tracking movement, so that could be part of the issue, along with the difficulty of tracking a longer focal length lens. I haven't tried the TC 1.4x(420mm) with moving subjects yet, which will slow down focus speed even more. Some alternatives...
200/2L/IS ($6000) - new sharpness king; replacement for the legendary 200 1.8L which was discontinued due to lead in the glass elements (dangerous to Canon workers, not to users). The 200 also has a low-light and subject-isolation advantage over the 300 2.8L. The 300 is $2000 cheaper and weighs the same (5.6 lbs.). If you shoot more at 200mm, then get this one. If you shoot more at 300mm, then get the 300 2.8L. The 200 + 1.4x extender is great, but not as good as the 300 2.8L (naturally).
300/f2.8L/IS ($3900) - Canon's next sharpest lens (along with 400/2.8L/IS). Legendary colors, contrast, bokeh, and focus speed/reliability. Negligible degradation with 1.4x extender; very sharp with the 2x. Fairly hand-holdable. Cons are price, weight (5.6 lbs), conspicuousness, and MFD of 8.2 ft (can rectify with extension tube). Canon has just announced a new version of this lens that is sharper, lighter, and has better IS - sounds ideal if you are like me and need 300mm more than 200mm.
400/f5.6L ($1000) - almost as sharp as the 300/f4L IS and faster focusing, but get this only if you usually need at least 400mm and have good light. Note that the 300/f4L IS + TC 1.4x is about as sharp as the 400/f5.6L, except at full-frame corners, but the 400 is going to be much faster focusing than even the 300 w/o TC, so you should choose based on whether you need AF-speed/longer-reach vs. IS/less-reach/macro/versatility.
400/f2.8L/IS ($6600) - 12 lbs; the best 400mm; tied with 300/2.8L/IS in sharpness; for pro sports photographers; tripod/monopod is required. Canon has recently announced an updated version like the 300 2.8L II.
400/f4/DO/IS ($5200) - only 4.3 lbs, but not any sharper than the 400/5.6L; it's too much money for IS and one extra stop.
500/f4L/IS ($5500) - 8.3 lbs (relatively lightweight), great for birding; could also use it for field sports with a full-frame body instead of a 300mm or 400mm, but wouldn't have f/2.8 speed; this and the 600/4L/IS are close behind the 300 and 400 in sharpness.
I recommend the The-Digital-Picture.com
for comprehensive reviews.