Fergus Millar brings up an interesting entry from 325 AD in the Codex Theodosianus which is actually relevant to the comparison. Damnatio ad metalla, or condemnation to the mines, was recommended as a substitute for being sentenced to gladiatorial combat (EDIT: reading the other posts, it's perhaps worth pointing out that it actually says "bloody spectacles," rather than gladiatorial combat. Others have noted that being a gladiator was perhaps not so bad, but I think this is getting at what you're asking. The point is that being condemned to the mines was perhaps roughly equivalent to being sentenced to death by spectacle), and Millar suggests that this was a "Christianizing measure." The "Christianizing measure" here is probably the elimination of the public spectacle, with the two sentences probably being viewed by the Romans as roughly equivalent. An entry from about a decade earlier in the Codex Theodosianus also links the two, forbidding facial tattooing of prisoners sentenced either to be gladiators or miners (one should, of course, simply tattoo both the hands and legs!)

Moving on, as others have mentioned, working in the mines was incredibly, horribly unpleasant. Damnatio ad metallum wasn't an immediate death sentence, but this also wasn't necessarily a kindness. Diodorus Siculus, a fairly early (1st century BC) source for this punishment (in the context of Egyptian gold mines), actually views this as one of the cruel aspects of it. His account contains this rather famous line: "Consequently the poor unfortunates believe, because their punishment is so excessively severe, that the future will always be more terrible than the present and therefore look forward to death as more to be desired than life." In this context, it was perhaps good for those sentenced to the mines that their life sentences were unlikely to be particularly long.

Likewise, if our (admittedly rather biased) accounts of the early 4th century persecutions are accurate, during this period damnatio ad metallum was often not the only punishment one received. Eusebius, referring to a metallum near and dear to me — Phaeno, in modern Jordan — relates that prisoners sentenced to the mines were often mutilated, as well. As an example: ". . . those with him, men who had shown the noblest firmness in behalf of religion, were condemned by him to labor in the same copper mines, command being first given that their ankles be disabled with hot irons," and slightly later, "He condemned three to single pugilistic combat. He delivered to be devoured by wild beasts Auxentius, a grave and holy old man. Others who were in mature life he made eunuchs, and condemned them to the same mines." (Note also the connection, again, to death by public spectacle.)

So, yes, being sentenced to the mines in the Roman world was likely one of the worst things that could happen to you. Being a metallurgical specialist, on the other hand, likely continued to be a pretty good thing.

I'm copying what I've posted somewhere else on the mines in Iberia under the Roman Empire.

Under the Roman Empire - exactly when a gladiator like Maximus would have been fighting - the majority of metals came from the Iberian Peninsula - Spain and Portugal. Pliny, in Natural History, wrote that "Almost all of Spain abounds in mines of lead, iron, tin, silver, and gold."

Silver was the metal of the Roman currency during the time of Emperor Commodus, the coin called the denarius, and thus the Iberian silver mines were incredibly wealthy. To own or operate one of these mines would have brought you massive wealth - according to Diodorus of Sicily, the sums would be "defying belief". Unfortunately I don't have exact numbers on wealth.

As to how they were worked, I have a number of primary accounts. I'll start with Diodorus of Sicily, from Historical Library. Typically aristocratic Italians would purchase a large number of fit and strong male slaves and ship them to Iberia to work the mines - they would dig deeps shafts with hand tools and attempt to locate seams of the desired metals. Diodorus states that the slaves would dig both "a great distance... [and] push their diggings many stades in depth and run galleries off at every angle..." However as time went on the mines were more and more controlled by the state.

Unfortunately for the slaves this was terrible, terrible work - if there is one thing you never, ever, ever want to be, it is a slave in the Roman mines, "for no respite of pause is granted them in their labors, but compelled beneath blows of the overseers to endure the severity of their plight, they lose their lives in this wretched manner, although certain of them who can endure it, by virtue of their bodily strength and their persevering souls, suffer hardships over a longer period; indeed death in their eyes is more to be desired than life, because of the magnitude of the hardships they must bear..."

Its not in this source, but I do remember learning that being condemned to the mines was a legal punishment, especially if you were a disobedient slave.

Diodorus also talks about the mines at Elba, in Italy, and here he talks of the process of actually smelting the ores. It is as follows: "now those who are engaged in the working of the ore crush the rock and burn the lumps which they have thus broken in certain igneous furnaces and in these they smelt the lumps by means of a great fire and form them into pieces of moderate size which are in their appearance like large sponges. These are bought up by traders in exchange and are then transported to Puteoli (a rich metal working city) or the other trading stations, where there are men who purchase such cargoes and who, with the aid of a multitude of artisans in metal whom they have collected, work it further and manufacture iron objects of every description. Some of these are worked into the shape of armour, and other are ingeniously fabricated into shapes suited for two-pronged forks and sickles and other such tools; and these are then transported by traders to every region, and this many parts of the inhabited world have a share in the usefulness which accrues from these products."

I also have an inscription for a mine in Portugal that details how very strict mining was controlled - there were a plethora of laws that basically said if you did not mine quickly enough your mining rights would be forfeit - explains some of the reason why it was still profitable to work your slaves to death - if you didn't, you'd lose the mine!

All my sources came from Naphtali Lewis and Meyer Reinhold, Roman Civilization: Selected Readings, The Empire, Vol. 11, 3rd Edition.