Qn 1: I have only been a Canon user since April (Pentax before that); I have a 5DII, with a Rebel T3i as back-up (comments below are based on use with 5DII)
1.17-40L - good enough for wide-angle work while I figure out whether my fondness for ultra-wide (in my Pentax days I enjoyed the Sigma 8-16mm at 8mm) is just a passing fad.
2. 20mm f/2.8 for when I want fairly wide-angle in low light (I prefer to avoid flash when possible). Haven't used it much, but it seems to do a nice job.
3. 24-105 L. The kit lens for my 5DII. If I'm taking just one lens with me when in urban tourist mode, chances are it will be this one; quite versatile, not too heavy.
4. 28mm f/1.8. Bought it as a fairly wide lens for low light, but I'm not much impressed with it for such a purpose - wide open in low light it's mushy around the edges. The 24-105 at f/4 does a better job at 28mm, frankly. May not keep it.
5. 40mm pancake. Bought this in part because I liked the Pentax equivalent so much just as an object (yes, a silly reason). The Canon pancake doesn't come close to that aesthetically, but it's probably better in every other way- an excellent lens, though I don't find myself using it often.
6. 50mm f/1.8. Too cheap to resist, and miles better than the 28mm f/1.8 wide open in low light. Don't use this focal length much, though - might consider upgrading to the f/1.4 if I did.
7. 50mm macro. Good for when I want to get more-or-less on top of a small object, and perhaps better overall than the f/1.8.
8. 70-200 f/4 IS. Almost as good as the 70-200 f/2.8 II but half the price and half the weight; I use this lens more than any other. Doesn't stop me from wanting a 70-200 f/2.8 II as well (and it would be as well rather than instead - I can't imagine lugging the latter on, say, vacation to Europe), even though I'm not sure how often I would see a difference in the results. Need to rent one again....
9. 70-300 non-L IS. Bought this before #8, assuming I wanted the extra 100mm. It's a perfectly fine lens, but crops from #8 at 200 look better than the images this makes at 300, and in every way #8 is a superior piece of machinery - so I'm not really sure why I'm keeping this....
[10. 70-300 L IS. I assume the copy I bought was faulty as some of the time the photos I took with this lens were less sharp than with #9, so I returned it. May eventually try another copy, but for now #8 will do.]
11. 85mm f/1.8; a remarkable bargain for a lens this good. Great for low light (among other things).
12. 100mm f.2.8 IS L macro. I use it less for macro purposes than as a standard lens that lets me get very close when I want and offers the advantages of IS, extremely shallow depth of focus, and lovely smooth backgrounds. That said, having recently rented a copy, I'm pretty sure I want a 135mm L too - even better in low light, even smoother backgrounds and, maybe, even a bit sharper.
Qn 2. I've only owned a dslr for a couple of years. Other interests - classical music, vintage fountain pens - eventually faded enough to free up sufficient funds to support this one as well.
Qn 3. Reading what others have to say is informative and entertaining enough, but supplement it with first hand experience. If, like me, you don't know people with equipment you're interested in and who are willing to let you try it, avail yourself of the various lens/camera rental firms. (And while you're about it, rent a full-frame body and test the comment you referred to about glass vs bodies....) It's interesting to compare different lenses of the same type and see whether the differences that matter to others matter to you; you may even be able to offset the wish-list you end up acquiring by ruling out lenses you thought you wanted!
