What do y'all use and why?
I'll just set aside the brand discussion stuff and go into the specifics of why I picked the camera I have now.
I use a Canon 1D Mark II. My prior camera (and still used as a second body) is a Canon 10D.
Here are the main reasons that I upgraded from the 10D to the 1Dm2.
1) Better sensor. The 1DM2 sensor had the second best image quality (behind the 1DsM2) of any mainstream digital camera at the time I bought it. The top of the line Nikons were comparable, but they were Nikons

:joking shudder of mock revulsion:

.
2) Shot buffer size. My 10D has a 9 shot buffer. There is nothing more frustrating that having your subject in the middle of a great action sequence and your shutter goes dead because the camera's buffer is full. Invariably, this happens at the times when you want to take the most shots (otherwise you wouldn't have filled the buffer in the first place). With the 1D, I moved up to a 20 shot RAW buffer and write speeds so fast that I've never hit the buffer limit. Even with the buffer full, I still get a shot every half second.
3) RGB Histogram. Once I learned how to properly use a histogram for metering, I was infuriated to learn that mine was only an average luminance histogram. At that time, only high end cameras came with an RGB histogram, so moving to a 1 series Canon was the only way I was going to solve that problem. Now I think you can get an RGB histogram even on base model DSLRs.
4) AF speed. The 10D was competent with auto-focus, but shooting kids is extremely demanding. The 1D has a very sensitive (works at f8) and fast autofocusing system. Only the Nikon D2Hs is in the same league for AF speed and tracking ability.
The body AF speed also works nicely with Canon's USM lens motors to make for the fastest possible AF system. Years ago, Canon switched from having the focus motor in the body to putting it in the lens. That adds a bit of cost to each lens, but it brings several advantages. The motor can be customized for the needs of each lens. It can also be quieter and faster. The AF system speed advantage is probably the single biggest reason that Canon dominates the sports shooter world.
5) AF Sensors. In addition to being much faster, I went from 7 AF points to 45. With only 7, I was often compromising on composition in favor of focus accuracy. Now I can pin down an exact spot in the frame as my focus point.
6) While not critical, I like having a high shooting speed. My 10D could do 3 frames per second while the 1DM2 can do just over 8. I've used that several times to ensure that I get exactly the shot I want (bat hitting the ball, explosion coming out of the canon, diver just entering the water). I've also used it for a few action sequences. It's a mode that is best to use outdoors, though, because it is just scary sounding.
7) Startup time. My 10D took more than 2 seconds to start up. The 1DM2 starts in less than a second. When I was just walking around and something photo-worthy suddenly appears, that 2.5 seconds seemed like an eternity. Again, I don't think any modern cameras have this problem.
8) Ruggedness. I'm not a gentle or careful person and I spend a lot of time in difficult (for cameras) environments. The Canon 1 series bodies are very durable and well sealed. Even thought it had a magnesium alloy body, my 10D would probably be dead by now if it had gone through the abuse that my 1D has taken.
9) Dual card capability. This actually didn't influence my decision at the time, but I've since grown to love it. After knowing people that have lost photos from card corruption, I now shoot in dual mode when I'm shooting anything important. In this mode, it writes the images simultaneously to two different memory cards. It's also nice to be able to use either CF or SD cards.
10) Portrait grip. This is actually a mixed bag. I almost never shot without a portrait grip on my D60 and 10D. Having it built into the camera made for a more solid feeling shooting platform. I have to admit, however, that it was sometimes nice to "go small" and take the grip off. It's not that I mind the weight so much; it's just that a 1D draws more attention that I sometimes want. If you walk around with a full sized DSLR, a relatively large lens, and a lens hood, you can almost guarantee that people will pick you out of the crowd as the person to take a picture of them with their little P&S.
I moved from a Canon D60 to a 10D when my D60 was stolen. The improvements that I recall going from a D60 to a 10D were:
1) Better AF system. While the 10D AF was still very limited compared to the 1 series, it was still a significant step up from the D60.
2) Rotation sensor. Back in "the old days", you had to manually rotate shots taken with the camera in a verticle position. If you mix modes like I do, it was a nuisance to have to select all the flipped shots and rotate them. I think every modern camera does this automatically now, but it was a nice feature back then.
At the time I made the purchase, Canon had not started offering "digital" lenses, so the larger sensor also made wider shots possible. The whole sensor size debate is incredibly complicated with lots of pluses and minuses on all sides. I suspect that in the future I'll be shooting with a "full frame" camera and I'll use whatever "crop factor" I want. However, it looks like the mainstream marketplace is moving to 1.5x-1.6x sensor equipment, so the full frame route will be noticeably more expensive. I suspect it will be something like the old divide between 35mm and medium format shooting only this time I'll find my self on the other side of the divide.
If I were starting from scratch today, I'd still probably drool and swoon over the 1DsM2 but would end up with a 1DM3 because too much of what I shoot requires speed more than it does the extra IQ of the 1Ds. If my subjects didn't scurry around so much, I'd also be tempted to get the Canon 5D. I think it's one of the best values in photography today. I'm tempted to get one as a second body, but I want to wait to see what the 1DsM3 looks like.