I have and use a Minolta Autometer IVf. It's accurate and well made.
Having said that the only use I have for it now is to measure studio strobe output so that I can work out lighting ratios.
I never use it for any other type of shooting. Back in the film days we were all in the prediction business. Now our business is review. Your camera's light meter will be extremely accurate for ambient light photography. And of course you should be checking the histogram to make sure.
I should also point out that a flash meter will not measure the light from the 580 unless the flash is on a manual setting. On E-TTL there are two flash pops, the first is to determine the exposure or to tell an E-TTL'd slaved flash how to behave. Then there's the "real" flash pulse, that lights the subject. The flash meter will be triggered by the first pulse and won't be ready to pick up the second. There is only one pulse when the flash is used on Manual - in any power setting - and the flash meter it will work fine for measuring that.
It has been suggested that a spot meter will give more accurate exposures in lighting situation where the most of the composition is dark but the subject is very bright. A performer on stage for example. But I've found that in a stage show the lighting changes so quickly that a spotmeter reading is obsolete before you can put the spot meter down!
If you are looking to explore ambient light photography perhaps you should jut be looking to tweak your exposure - as indicated by the histogram - so that the highlights and the main body move closer to the right side without clipping. This should improve the noise, and you can determine this by experimentation.
So unless you are using studio strobe lighting, I'd forget the meter.
"There's never time to do it right. But there's always time to do it over."
Canon 5D, 50D; 16-35 f2.8L, 24-105 f4L IS, 50 f1.4, 100 f2.8 Macro, 70-200 f2.8L, 300mm f2.8L IS.