There's nothing wrong with today's zooms, thats about all I use. Anyway, two things you need to consider. First, which camera format are you using? A 24-105mm zoom (which I have) gives you a different effective range on a 20D with its 1.6x factor than a 5D (which I have), which would give you the real zoom range. What are shooting? How far do you need to zoom? What kind of f-stops do you like? How heavy do you want your lenses? I have the 24-105mm f4 and the 70-200mm f2.8 because I like constant f-stops, but they do have some weight to them (especially the 70-200mm). You can get lenses in the 50-300 range, but they have shifting f-stops, the largest of which might be f4.5 or f5.6. Can you live with that? (I can't). BTW, I have long-time pro friend who uses the Tamron 28-75 f2.8 lens with a 20D and says its tack sharp and weighs almost nothing.
Regarding push-pull zoom. Personally, I find it much easier to do that trick with a twist zoom than a pull zoom, but that's just me, and I haven't done it lately.
Andy Pearlman Studio