Wedding receptions are often very, very dark places, i've seen receptions with ambient light levels that need ISO3200/F2.8/0.5sec to expose correctly! Even with a modern camera and fast primes with ISO6400 and F1.4 you'd need 1/16th, which personally I find too slow because of camera shake and subject movement. The quality of light isn't always very flattering, with a mix of candles, tungsten bulbs, and fluorescent lights. Because of that I choose to light most receptions, overpowering the ambient light, though sometimes I set up to enhance it using gels and lower powers. Beware that a cto gel to match tungsten can lose a stop of power. If you want to take an ambient shot to show the atmosphere it's easy to flick back to Av mode and ramp up the ISO to try to get a decent shot.
I typically use two Canon Speedlites (550/580) on these stands, with Canon CP-E4 battery packs to cut recycle time by 2/3. The lights are triggered by Skyports, with locking PC cords from www.flashzebra.com so I can have a flash on the hotshoe as well. I use this cord to connect the skyports to the flashes, and this bracket to connect the flash to the stands. I used to use Sunpak flashes, but they didn't recharge quickly enough for me, and with a home made battery pack one of them almost caught fire at a reception, so I switched to all Canon for performance and reliability.
If the venue has white ceilings I usually use between 1/4 and 1/2 power on the flashes, which gets me ISO400-800, F2.8 - F4.5, depending on the venue. If the ceilings aren't white I point the flashes directly at the head table, usually at around 1/16th or 1/32nd power, which gives me ISO400 F5.6 or so.
As a quick aside - for location lighting I use a Morris Soft Box which is about 20", with the speedlite, battery pack, and skyport mentioned above. My assistant holds it, it's quite light.
Here's a few examples.
#1 - this shows the setup
#2 - this shows the type of lighting I get with the setup above
#3 - a cake cutting setup
#4 - a whole room shot, to show what two strobes can do. I probably used a little bit of on camera flash too.
#5 - this room had a black ceiling and green walls. I photographed the head table using direct off camera flash. ISO400, F5.6, 1/125th, flash power probably 1/16th or so. I used 70mm so each person was about the same size but still keep some DOF, at 17mm the guy in front would be huge and the girls at the back tiny. Beware harsh shadows if you shoot direct, like the silhouette of the person on the front of the head table's tablecloth.
People have asked for photos of how I set my camera up, so here they are
Camera with custom locking PC sync cable fromFlashZebra.com
Showing the connection to my camera
It's mounted to the flash head with velcro on the flash head and the transmitter. Ideally I mount it with the antenna away from the transmitter body, pointing up