Chris,
Your may remember my PM a few months ago giving a good report on my 50D 15-55 f2.8. I may have mentioned the dust problem too.
IMHO here is the best Canon review site:
http://www.the-digital-picture.com/R...ns-Review.aspx
You will find many other sites with comments about front element dust.
There appears to be porr or no sealing to the area behind the front element, so the large change in volume when zooming creates an ideal pump. I have not come across a retrofit seal mod, but if that were possible, I am sure naturalists would go for it big time.
My 17-55 is now over 12 months old and has zero internal dust.
Its all about personal "camera equip hygiene" and being aware of this issue.
Yes this lens has very good optics, the above reviewer and others quote it at "L" quality without enviro sealing.
This lens is standard fit on my 50D and gets the most out of the increased pixel density of the 50D. My other zoom at present is a 70-300 f4, a bit soft at 250-300, but otherwise great on this camera and also manageable weight and balance.
The Canon EF 70-200mm f/2.8 L IS II USM would be my other purchase for this camera, but at AUD 3000 at reputable OZ stores or as low as AUD 2000 on ebay for the original version. Heavy and expensive but an unbeatable lens in this zoom range. The non stabilised version is a comparitive bargain, but once you have used fast stabilised optics on a high-res DSLR its hard to live without stabilisation.
Currently researching modding to clip-in filters.
Cheers,
David