Unless I missed it, you have never discussed what type of photographs you take mostly. If you're into landscapes, a wider angle than 24mm on an APS sensor will come in handy. On the other hand, if you mostly shoot street photography, it is likely that the 24-105mm range will be far more useful to you. Both of these ideas center on the idea of only one lens, however. There is not now, nor is there likely to ever be, a high quality lens that covers all of our photographic needs. My suggestion is to examine your photos and find what focal lengths you tend to shoot and go from there.
sorry Carl and fastcat for my carelessness. i firstly wanted this lens for travel but the trip was postponed to late Oct it can ever be made. I mainly want this lens for landscape and architecture (but i can't afford TS-E lenses). street photography too but i believe 55mm on 20D isn't that bad for my street need (88mm equiv), of cuz, 85 or 105mm is more handy.
so what say you?