Boots are a specific case. You can't have more than one movement buff from any kinds of boots at all.
As for other items with UNIQUE passives, as long as you don't have the exact same item, they all stack on top of each other.
Apparently, there are other exceptions as well. From the wiki:
Unique effects that do not cumulate among differents items include Movement Speed effects from boots (only the best effect applies), proc effect of Sheen, Lich Bane and Trinity Force, and the mana gained from Tear of the Goddess, Manamune and Archangel's Staff.
Rammus' great strength comes with the element of surprise. If Rammus isn't in your face, and he isn't powerballing, then he's not scary.
First, the general Powerball gank-tip: start charging it before you actually go in to gank. It takes some time to get spinning, but lasts for quite awhile. If you just press Q and charge on in, you give your opponent precious escape-time. Figuring out the timing on this is critical.
Second, wards. Namely, destroying their's. Rammus late-game can be something like Fiddlesticks; if you can get the jump on them from a surprising position, you've really got something. Have an oracle's on your team (either you or your support) and clear out those wards. Leave them guessing.
Third, scaling MR blues. Rammus needs MR, and these provide a healthy chunk.
Fourth, effective team communication. When Rammus goes in, he goes in. He isn't getting back out. If your team is not prepared to jump in after you, you're going to have a bad time.
Fifth, make use of your early-game advantage. Rammus is FRIGHTENING during the laning phase. If you manage your Powerball correctly, and your lanemate has any CC at all, you're pretty much guaranteed a kill. Gank frequently.
Sixth, give your lanemates the kills, whenever and wherever possible. Don't ever let a kill get away, but if the opponent is certain to drop, give it to your lanemate. They can put that extra gold to much better use.
Best Answer
Accurate as of May 2013
Aegis of the Legion is a very cost-effective item, and it now upgrades into Runic Bulwark, which is slightly less cost-effective, but provides more statistics. For just the item owner, with the aura hitting no allies,
Aegis is 107.32% gold efficient, and Bulwark is 96.56% gold efficient.Below is the math, in terms of gold efficiency. I've used the cheapest items available for each statistic to calculate the equivalent gold value of the final item. Obviously if you're comparing against more expensive (and usually more efficient) items, the gold efficiency of the Aegis and Bulwark will fall.
100% gold efficiency means the item is granting as many stat points as if you had spent the same gold on the raw stat items; A higher gold efficiency means you're getting more stat points.
As for the stacking portion of your question, its best handled in a separate question. The short version: If you own the item, you can have double auras, no more; everybody else gets a single aura.
Gold Equivalencies
36 gold per 1 health regen20 gold per 1 armor20 gold per 1 magic resistance2.63889 gold per 1 healthAegis of the Legion - Gold Efficiency
Now, if the aura hits 1-5 champions:
The item's total cost is 2150g, so the item is gold efficient for just the item owner.
Runic Bulwark - Gold Efficiency
Now, if the aura hits 1-5 champions:
The item's total cost is 2950g, so the item is 96.56% gold efficient for just the item owner, and is fully gold efficient if the aura is on at least one ally champion.