… Does anyone do this? Is it practical/comfortable?
And, mini-poll, just out of curiosity: what size do you wear?
I have done it and it can be both comfortable and practical, but I would offer the following caveat: Most of my experience wearing a metal ring for an extended period has been while naked. When clothed experiencing some discomfort is possible. Tight jeans, for example, are not a particularly good choice and you are liable to find that you're constantly having to "adjust" to avoid some pinching of the skin. With loose clothing, such as boxers and dress or casual slacks, that problem would be much reduced.
Having heard horror stories such as notN2pussy eluded to, when I ordered my own metal cock ring I went oversized. Even though I have always been able to get my large balls and nearly 8" by 6+" cock into and out of a 2" ring my metal ring has a 2-1/8" i.d. It is still tight enough when I am erect to enhance my girth and hardness and has a comfortable, heavy feel when flaccid.
Also, I would be wary of Dazzer's recommendation that you go with a chrome ring. That, because chromium is a very common agent in creating allergic contact dermatitis (ACD) and even if you have no current allergy to chromium regular use of a chrome cock ring could very easily result in that allergy. It would be a shame to have a high quality cock ring that you ultimately could no longer use. For that reason I highly recommend going with stainless steel. The SS rings are among the most expensive ($20 and up) but I believe they're well worth it. My own skin is particularly sensitive to metallic induced ACD but my SS ring has never caused me a problem.
Straying a little from your original quest I'll add that my own all time favorite cock ring for extended wear is one of 100% silicone (pictures attached). Because the silicone is stretchy a good snug fit is possible without any fears of it having to be cut off in the emergency room. It doesn't have the nice weighty feel of the SS ring but it is very comfortable for extended wear.
Whatever you choose in a metal ring proper sizing is key. If you can find a variety of cheap plastic rings to experiment with different sizes that would be a good way to ensure that your metal ring will serve you well.
SS&Si