Do you take Supreme Commanders with an eye to having a rock-hard elite detachment or fast-moving leader who influences the battle at opportune points? The different races seem to have different styles- orks are more likely to have an iron-fisted mob of nobs or jump packs but they aren't particularly fast unless you mount them in battle fortresses; Eldar will usually have a fast-moving, elite force of Aspect Warriors that strike deep and brutally; while Space Marines and Imperial Guard are smaller and less likely to be major contributors to a combat unless its for the reroll value.

What's your ideal Supreme Leader detachment? How have you used it?

Imperial (and CSM) supreme commander detachments are definitely smaller, but I don't think there is any limit on how many you can take (as there is for Tyranid, for example). For an Imperial force, I might take three or four in a larger game and attach them to my detachments as needed whenever they need to take a certain objective. Even if Space Marines do not get much benefit from stubborn, the reroll in combat granted by supreme commanders is invaluable, as it allows you to conserve your precious fate cards for the fights that catch you by surprise (or burn them for secondary abilities, like Psychic Blasts to assassinate particularly bothersome stands; perhaps even enemy supreme commanders!).

I have to admit, I haven't ever thought of taking multiple supreme commanders because it says "up to one" in the army but going back I see that means in the one detachment.

Yes I never interpretid it that way either always considered that you could only have one in the army but looking at it there's nothing to stop you having more than one supreme commander detachment. You could always look at them as battalion commanders or such the next level of command above the detachment company comanders.

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I have used them sparingly. Some are pretty expensive and rather small. However, I am big on some kind of organization, and believe every army should have a designated force commander on the board. I really dont like individual detachments running around with detachment HQs but no force HQ, so I prefer to have one, but they are usually the last thing I consider buying since they arent necessary in game terms.

Also, force commanders main roll is commanding, and while the 40k world is sort of Hero-hammer, I tend to leave whatever Supreme Commander I do have in the rear. I only bring them out when its to their advantage kind of late in the game. Its just where I feel they would realistically be.

I haven't seen many games (including non-GW) where there is a pretty important leadership reason for having a overall commander, nor any side effect for losing one during the game. I mean, losing a army's HQ would have a major effect on the force if it were lost in real life.

Back in AT/SM1, when infantry rules first started appearing in WD before SM1 was released, detachments had detachment commanders (some units having more than one - like a backup), but for extra cost you could turn one of these commanders into a force commander for like 50 points I think. It was a pretty big deal, because you could only give orders to units who had a commander, and if it were killed, there was a good chance that detachment would sit still, or fall back. The force commander allowed you to give orders to those units without a commander once a turn. Lose the force commander, and you lose that ability too.

But that rule disappeared when SM1 was released, and I don't know why. It one of the coolest rules I have seen in Epic, and I feel like a better representation of HQ units than in most GW games.