Well, as this isn't really about "Aviation Photography", I thought this might be the proper forum. By all means, move it if I'm mistaken.
I recently jumped onto the DSLR bandwagon when I snagged a 450D/XSi kit on clearance for $450 shipped from Vann's. I was originally looking for a P&S with manual controls (G11?) to hold me over until a full frame DSLR made sense, but this ended up at least as cheap as any P&S I was considering. I have $200+ in B&H Photo gift cards sitting around, so I might as well make a purchase before the currently ongoing Canon rebates expire on July 10th.
My 450D/XSi kit came with an 18-55mm lens that is limited, though my particular example is not quite as bad as I had feared. My personal experience is that sub $1k lenses can vary quite a bit in production quality. I bought a 50mm 1.8 simply out of obligation due to the sharpness and $99 price. I've actually found that the "effective" 80mm focal length can be fun, though I rarely used an 80mm focal length with film - probably due to the fact that it was at the crappy quality end of most kit zooms.
Anyway, to the point...
In light of the above I'm looking for my next lens.
I already have a good bag, tripod, filters, etc from an old system. I've also bought the grip that holds 2 batteries simultaneously, an extra pair of batteries, screen protector, and Ben Long's companion book. I'm not new to photography, so I don't expect that my money is better spent on "something other than a lens" at this point but I'd welcome suggestions to the contrary.
All prices below are shipped, no tax, after (instant) rebate, non-inclusive of my gift card credits. My thoughts are included below as well. I have no specific desire for a lens of given focal length as I enjoy many types of photography, though I haven't done much macro work. Based on other reviews, I'm currently of the understanding that all of these lenses are great (or at least great for the price) - I simply can't decide which way to go on my next purchase. This site should have enough photographers around that I hope to not be short for opinions.
I'd say that these are somewhat listed in the order in which I am likely to buy. It probably makes sense to go for some versatility so long as quality isn't too badly compromised. If I get no/few responses, I'll probably just grab the 70-200 f/4 non-IS for less than $400 out of pocket by parlaying the current rebate with my gift card balance.
Canon EF 70-200mm f/4L USM: ($594)
This is probably the best deal going. I'm not sure that I'd miss the IS, at least having not used it significantly (yet). I'd rather have the f/4 than the 2.8 given the money/portability hindrances associated with the faster 2.8 lens. I also shot with slow non-IS lenses in college, so I'm sure I could manage with this one. The thought of a 320mm (equivalent) f/4 lens with nice optical performance is appealing. This lens is currently eligible for a $45 instant rebate reflected in price above.
Canon EF 200mm f/2.8L II USM: ($769)
For pure image quality, this is probably the best lens on my radar. I had an old Nikon zoom that I'd often use at 300mm (and not just because that was the max reach in my kit). As a result, I think I'd find some legitimate uses for this lens at a similar "equivalent" focal length. However, it seems like this may not be the best use of my money compared to the 70-200. This lens is not rebate eligible.
Canon EF 17-40mm f/4L USM: ($700)
This is a great "one lens only" focal length for a 1.6x crop sensor. I'd probably have bought it already if it weren't for my initial impressions that the 18-55 IS kit lens is "tolerable". As an aside, it's also useful on a full frame AND 1.6x crop camera body - Something I wouldn't say for the excellent 24-105 Canon given my shooting habits. 24-105 is a great range on full frame, boring on a 1.6x (IMO). This lens is currently eligible for a $50 instant rebate reflected in price above.
Canon EF 300mm f/4.0L IS USM: ($1269)
Certainly not the most versatile lens in this lineup. However, this (or the 200mm + 1.4x) is the fastest ~300mm telephoto I could realistically buy without calling off my upcoming wedding. This does have the advantage of deep reach (on the 1.6x body) with adequate aperture and IS (something I'd be more interested in at this focal length). For what it's worth, opportunity cost is less on this lens due to the lack of a current rebate and my expected limited use.
If I were going to step up to the IS version of the 70-200, I'd be more likely (foolishly or not) to just buy this prime.
Canon EF 24mm f/2.8: ($339)
Because I enjoyed the 35mm focal length in my film days, I was thinking that this would be my next purchase for "point and shoot" type purposes. As I tend to leave my camera laying around during family functions (free for anyone to use), I wanted a wide prime like this to "protect casual users from themselves" and force people to step closer rather than crank a kit zoom to f/5.6 indoors! However, after looking at several review sources, it just doesn't seem like this lens is that much of a "gem" in the Canon lineup. I always thought the cheaper Nikon primes were better than the Canon counterparts (I'm not asserting that one is better overall than the other as I don't want to start that fight).