I just got the Prometheus Alpha Bamboo Pen in brass last week. As soon as I held it in my hand, it blew me away. I am a bit of a pen nerd and have pens that are much more expensive but this pen keeps up with all of them. The precision and attention to detail makes it feel like the pen was made by hand, one at a time. I was on the fence about the Bock nib but it turned out to be flawless. If fountain pen aren't your style, the rollerball section is just as impressive.
I also have the Karas Kustoms Retrakt in copper. It is considerably cheaper but you can tell the quality is not there. The attention to detail is lacking and the precision is not quite there. You do have the option to use many different types of refills though. Not a bad pen, especially for the price. Maybe something to consider if you lose your pens often or like the convenience of a click. I will say that the click mechanism did break on me and I had to buy a replacement. I heard that the early models were the one pens that were affected though.
My first pen in this catagory was the Hinderer Extreme Duty Modular Pen in titanium but they make them in copper and brass. I never liked it from the beginning. Too many threads on the cap and the cap is too small. Plus the clip is terrible. He has a bunch of different models now though.
I am interested in Matt Martin's custom pens but that will have to wait until I get some funds again. They are also some of the few pens that are made in bronze.
Something to consider, the copper is going to age beautifully with the most colors but it will be the most fragile. I dropped my copper pen once and it made a small dent. I had to reshape it a bit so that the pen could function again. The brass should be a bit stronger. But the bronze should be the strongest. At least from what I have been reading.