I have used both the 580 and the 730 here's my take:
The 580:
1. Very precise with target location I.D. If it vibrates, that target is directly underneath the tip.
2. The depth is probably about 3" as Craig mentioned.
3. The on/off for the 580 is more responsive than the 730, making for quicker actions.
The 730:
A little bigger and bulkier, but does cover more area than the 730. However, because of its size, it doesn't allow for the nice pinpointing of the 580. Depth is a little less than the 580 as well I feel. The ligth is nice if you can't always feel the vibrations, the light pulses as the detector vibrates.
My uses for these mini-detectors have been:
1. Very useful for fanning and digging targets out of tight rock spaces and underwater honeycomb rock. This is even more so when you use a detector without pinpoint mode as you might have 3 or 4 holes under 4" of sand and the target is in one of those 4 rock holes.
2. I found a use for it in some of the deep powder sand where the sand collapses back in as you scoop. Here I have the pinpointer in one hand, and scoop and move sand with the other hand, getting my hands and the pinpointer deeper and deeper. As mentioned, it only vibrates when the target is directly under the tip so you know where to go. Of course you would have had to find the target with a full size detector first.
3. The 580 was also useful for me when I was around some tight large rocks in the wet sand. A scoop was completely useless for this so I found the target with the Dualfield, then would use the 580 to pick it out between the rocks.
4. I did try to use the 730 solo on some tide pools and found a nail and an ironladen rock, however I think if there was any gold there, the 730 might not be responsive enough to pick it out of the 5" deep sand in the tide pools. Also feeling for vibrations while bumping into rocks is kind of hard as well.
One thing that should be mentioned, but makes logical sense: It is a metal detector, so it will have a frequency. I found we couldn't use the CZ-21 or the Dualfield while the pinpointer was also on, the threshold is too unstable.
They are very rugged, and waterproofing is good as well. Making sure it doesn't come on in your bag is a little difficult, but its nice there are no buttons to worry about. The tilt function works good when you're trying to cut it on/off.