I shoot weddings and own and use the 580EX, 580EXII, and a Metz 54MZ4-i, with various modules. I use them on a 20D, 40D and 5D.
1. The cameras I have are all ETTL II. I hate Evaluative flash metering because it is hard to predict and does things like underexpose in a big way whenever there is anything white in the frame. Your 1 stop underexposure across the board is normal. It also practically shuts down whenever it sees a specular reflection in the frame, such as shooting into a mirror, or when there is a metal, polished object in the frame. I use Averaging flash metering which is easier to understand and predict. If you are used to auto thyristor flash metering, it is similar.
However, the metering pattern in the camera (used for both flash and ambient) makes a big difference in flash results. On the cameras I own, the metering pattern is a 7x5 grid. On the 1 series cameras, the metering pattern is more like center weighted averaging patterns.
I currently use the 580EXII on my main camera (5D) because of several factors I don't like about the Metz 54, which I prefer for metering only. I just learned to deal with the underexposure.
2. On my cameras, if you have flash comp set on the body, it is overriden by any flash comp set on the flash (not additive). If the flash comp on the flash is 0, the flash comp on the body (if any) takes over.
3. With the Metz 54, auto thyristor comp indeed works. However, every so often, it will suddenly let go of the comp and revert to 0. I also asked a Metz (Bogen) rep about this. He gave me basically the same answer--flash comp on the flash was not designed for auto thyristor mode. Also, sometimes it will go crazy and do a full blast.
4. With the Metz 54, the ETTL implementation is an emulation. The ETTL sensor (don't even know what it does) is located in the lower left corner of the flash head reflector, as you look at the flash head. When you put something over the flash head, like an OmniBounce, it overexposes. The ETTL emulation is slightly better than Canon's implementation of ETTL (re underexposure).
5. The Metz 54 does not recycle as fast the 580s, which is an annoyance for me, shooting weddings. It is more fragile than the 580s. I've had it in for repairs on several different things a couple of time. It is in for repair right now, although this last was my fault--broken foot). I've fried the flash tube 2 times.
6. I use the 3102 M4 module for Canon, but used in auto thyristor, you are limited to manual camera modes, which is fine, but I do switch to AV or TV sometimes. In TV, auto thyristor does not work at all--AV is OK re aperture, but the shutter speed goes to the sync limit. Not great.
However, a little known fact is that the 3102 M (1 or no number) module allows you to use auto thyristor with Canon in AV, TV and even program. Metz auto thyristor is great--there are annoyances, though, and nothing is perfect, but the system is predictable because it is easy to understand.
7. With the 580EX II, be aware of the problems associated with the foot. The foot can prevent good contact with the hotshoe, causing the flash to go full blast. The foot can become loose over time and you can go in there and tighten it up. Look up the blog of a photographer who gives instructions (Shimworld + 580EX in Google). Or the hotshoe plate on your camera can become loose, which you can tighten.
Also, the 580EX II has thermal overload protection built in, and you can't turn it off. I've run into that a couple of times--all at the worst moments. I now use nothing but Sanyo Eneloops with a smart charger. They seem to do better re hitting the thermal threshold.
8. I use the 580EX II primarily because I can use high ISO and it recycles fast. I have the custom function set to use the big dial to compensate. I've long gotten used to the underexposure and just deal with it.
Here is my review on the Metz vs Canon
Pros
1. Auto has excellent consistent exposures.
2. TTL is very close to the mark.
3. LCD screen is lights up with any button that is pushed. I love that feature.
4. Menu system is easy and very intuitive.
5. Audible warning for potential underexposure.
6. Audible warning for correct exposures.
7. You can access key settings like flash EV & ISO through the camera info screen (if your camera has this
feature).
8. Data transfer between camera and flash for ISO, aperture and lens zoom.
9. Low battery warning indicator.
10.Good battery life when using high quality batteries.
11. Good recycle time. Same specs as my Canon 580 EXII.
12. Auto Zoom from 24-105.
13. Manual flash range = 1/1 to 1/256. Canon = 1/128.
14. Great colour - whites are clean looking.
15. USB port for firmware upgrades.
16. Extended zoom feature.
17. Crop adjustment for crop cameras.
18. Metal foot - upgrade from AF-1
19. Master and Slave control
Cons
1. Instructions are not easy to read.
2. Although the intsructions are not the best the menu system is easy to use. It takes on average one or
two extra steps than my Canon. At least I can see what I'm doing now.
3. No HSS in Auto mode but is there in TTL.
4. No EV in Auto mode. For EV you need to switch to TTL.
5. Foot lock design is not as good as Canon 580EX II.
6. Flash head rotation and tilt is not as fluid as Canon.
7. Flash head release button flush with body and not as easy to press as my Canon 580EX II.
8. Flash head does not rotate 190 degrees on both sides. 120 to the right.
9. Not weather sealed.
10. Secondary reflector is just to bright to be really useful.
11. No sync port. Not a big deal to me.
12. Cannot be used with very high ISO whereas Canon can.
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There is nothing worse than a sharp image of a fuzzy concept.
Ansel Adams
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