Sorry for the short descriptions and lack of detail on the newer cameras. Just haven't had time to keep this up to date.
This is the last pocket camera made by just about anyone that still has an optical viewfinder. So you get all the advantages of fast operation, reasonably decent software (at least it can sort of focus on red flowers sometimes -- older cameras never can; and it doesn't blow out bright areas like skies as much as older Canons) and very small size, without having to give up the viewfinder. I thought I'd left it in a hotel room once, and was crushed to find they're almost unobtainable even on the used market. I was very relieved when I finally found it deep in a suitcase.
The battery lasts forever. I carry a spare (they're small) but I only have to swap every couple of months, seems like.
Like all Canons, it can't do the USB storage protocol, so there's no point in plugging it in to the computer. I just take the SD card out and stick it in my card reader.
When I finally decided to get a DSLR, ironically, I chose a Canon because I wanted to use my old Nikon lenses. Nikon has orphaned the old manual focus lenses -- they screw in to the camera bodies physically, but the camera can't meter properly with them. Or so everybody says. But you can get a cheap adapter on ebay that lets you use an old Nikon lens on a Canon DSLR, strangely enough. And it works, though metering is a little inconsistent.
I love the camera. I bought it intending to use it in all manual mode -- I hate autofocus, it's always focusing on the wrong thing and you have to fight it endlessly. Guess what? That's not true with the Rebel. You can set it to focus on whatever's under the center spot (I wish they'd offer this for pocket-sized cameras!) and it actually does it. Fast. It's a joy to use. Although it's still big and bulky, like all SLRs, so I still carry the pocket camera a lot more often.
Irritated by the Canon A540, I was really nostalgic for my old stolen Minolta. So I hopped on ebay and got one. I'll still use the Canon, but it's nice to have the option of a tiny, solid camera that takes great pictures with no hassle.
My beloved Dimage Xt was stolen (I was careless) and the C-730 is too big to lug everywhere, so I needed a new portable camera. Unfortunately, it turns out that nearly all camera makers have stopped offering optical viewfinders! Since I take the vast majority of shots outdoors, often in bright sunlight, a screen is not an adequate substitute. So my choices were limited to the manufacturers who DO offer viewfinders: Canon, Sony, and a few HP and Kodak cameras that I don't really trust. Unfortunately the Sonys and the pocket-sized Canons can't do exposures over a second. So I opted for the Canon A540, which is a terrific deal in terms of features per dollar.
You pay for that, though: it has the typical clunky Canon UI, complete with bizarre and unpredictable details like which parameters are and are not remembered at power on. Flash setting is remembered, a good thing because otherwise I would have returned the camera -- this is why I didn't even consider the current Canon Elphs; macro mode is not remembered, which I don't mind a bit; the two that really bug me are drive mode and metering mode, so basically I can't use multiple shot or spot focus modes because the times when I need them (look! a coyote!) are precisely when I don't have time to fiddle with menus.
Also, its screen, though large, is so poor that it's impossible to tell whether a shot is good even by zooming. And it has some of the same problems with blown-out highlights that the S20 did, though it's not nearly as bad. And it tends to make poor exposure choices in low light. Oh, yeah, and it's USB camera mode, like the S20, so it can't be used like a disk: you have to use special camera software to copy anything from the camera to a computer.
Its hardware isn't very good either: at just under five months old, the lens cover is failing, and often doesn't open all the way.
And I'm rather happy I don't use Windows, after learning that the Canon Windows software does things like storing your real name, without asking you, into the camera's memory, where it's impossible to delete it later if you ever lose access to the Canon software or switch to a platform other than Windows. See this Steve's thread for a discussion of that problem.
On the plus side, it goes forever on 2 AA NiMH batteries, has a 4x optical zoom and theoretically lots of manual control, though don't expect to do much with the manual focus on nearby objects because it refuses to focus manually in macro mode.
Used mostly for bird and wildlife photos, because of the long
lens. Otherwise, it's rather bulky and heavy.
Supports usb-storage, plug and play with linux.
Tiny, lightweight, yet still decent at low-light shots (which is why I chose it over the lovely Pentax Optio S). So easy to carry that I've ended up using it for most of my photos. It takes great photos (makes very good decisions about exposure and okay decisions about focus) and can take long exposures up to 4 seconds. But the neatest feature of this camera is that it has no separate macro mode: if you want to take a macro shot, just shoot it and it focuses properly.
I'm quite happy with it; I just wish it started up quickly, since that was one of its marketing points and it didn't live up to the hype. Usb-storage, plug and play in still camera mode; it also has a webcam mode, which I haven't managed to get working with linux.
The USB works with gPhoto2, but gPhoto2 is a pain for various reasons (it's hard to "delete all images on the camera" and it names everything in upper case) so I used a USB CF reader. I was never happy with the picture quality from the S20, and the 2x zoom was very limiting. I was happy to move on.
Serial interface, so I used a smartmedia reader for this one. Excellent camera, good lens, good metering, but it's rather bulky for only a 3x zoom.
Relatively small and light, manual control, interchangeable lenses! I wish someone would make something like this in digital!