This probably the most handy lens in the Canon line despite its highly critized push/pull zoom that I find faster than the ring zoom system and the exagerated complaints of being a dust sucker. The major shortcoming is the less than excellent image quality particularly the 400 mm range. With the newer pro cameras delivering great images at higher ISOs, I use this lens at f 8 to obtain better photos but it still short delivering quality images.
Canon surely will have a winner if it replaces this lens with one delivering better quality images using the current push/pull zoom and a fixed f4 or f4.5 speed.
This lens occupies a unique space in the lineup and has little competition. It may be older, but that may explain its relatively low price. I hear it still sells well. If they do replace it, it will be with something with all the modern technology but expect the replacement to be considerably more expensive.
The 100-400mm f/4 with built-in 1.4x extender was the 2012 Olympics and the rumor mill has it schedule for release this year. But, it will cost a little more than the current 100-400mm.
Actually it is a 200-400mm f4L IS w/1.4x built-in converter. Unfortunately the rumored price is said to be around $13K USD. A little more than a lot of us have to spend on a single lens...
As for the 100-400 being a "dust pump", it is. I own both it and the other "dust pump", the 28-300/3.5-5.6L IS. Heavy use of either lens can deposit quite a bit of dust on the sensor. It seems to be worst on full frame but there is a lot more surface area than APS-C. I don't know if it actually sucks in a lot of the dust but the air pulled through or pushed into the mirror box disturbs any dust particles that are already lying around in it. Once they are disturbed and swirling around in the body, the sensor is just as likely a spot for it to hit as anywhere else. You may think that there's not a lot of dust particles in your camera to be disturbed but you'd be surprised. Unless you're in a "clean room", every time you change a lens you have an opportunity for dust entry no matter how careful you are.