This nice armor is supposed to compensate the deficiencies of its "heavy armor" status with a constant muffle effect while sneaking (it's still better to wear light boots, bracers and helmet, though).

The enchantment will stop working the moment a hostile creature/NPC is around and it will be replaced by a 'ward' type of effect which will cause a small, costant damage to nearby foes.

A major problem I noticed happens when potential threats are not full hostile but in "search mode" (eye half open): the muffle effect will be deactivated although the armor won't immediately activate the other protective spell. Most of the time, this premature disabling of the muffle effect will make my character plainly visible to nearby enemies because the penalties for sneaking in heavy armor will be back in full effect.

I believe this is an oversight in the design of the armor, not to mention that the 'cool' shadow is also quite annoying during combat or while using the bow to take long range shots. There's a MOD do disable the graphic effect but I don't think I've seen anying to fix the odd behavior of the armor which makes it much less useful than it appears at a first glance.

So, nobody opted to use this unique heavy armor in game? (different and better in appearance than the plain ebony armor).

Is it truly useful for the occasional stealth kill or for moving undetected?

I used it to test out a mannequin. Wound up with two of them due to a bug and a mannequin wearing one. Other then that I stick to light armor.

When I want to pass undetected I take off everything from my character. With 100 in sneaking, I had no trouble completing all the thieves guild quests without resorting to violence or subterfuges at all. Having maxed out also light armor, I wanted to invest in heavy armor for the most thick fights.

Not wanting to abandon stealth completely, I decided to try the ebony mail. Unfortunately, its enchantment seems a bit useless if it wears off the very moment the enemies will start looking around.

There are two ways for enemies to be alerted; sight and sound. You'll only be completely detected, however, by sight. The muffle enchantment only effects sound, not sight. This is effective until someone looks in your direction, so it only really helps until an enemy is alerted.

What you say makes a lot of sense but...

-The ebony mail also has a visual effect that turns the player into a walking shadow. It wouldn't make a lot of sense if the enchantment only suppress sounds.

-If I take the clothes off and stay in the same spot (no moving around) I will be harder to spot. I've tried this experiment several times: I sneak to a dark corner of a room containing an enemy, I shoot an arrow at him and see the reaction. If I'm naked the guy will run to me, stop in front of me but the eye icon will remain half closed. If I'm wearing armor, the eye will open some more, sometimes they are able to spot me. This makes no sense! Either they see me or they don't... apparel shouldn't make a difference when I'm not moving. This, unless color is taken into account but I doubt Skyrim reaches this level of sophistication (not to mention that a black armor will be at an advantage compared to some light brown leather outfit).

I didn't actually know about the shadow thing. I'm betting that's only a graphic effect. You're right, though, that wouldn't make a lot of sense except to look cool.

That test is also strange. I was under the impression that armor only effects sound. http://www.uesp.net/wiki/Skyrim:Sneak I wouldn't be surprised to learn that it makes you easier to spot, but no one has picked up on that yet as far as I know.

I have heard that the Ebony Mail aura makes it difficult to sneak right next to someone since it damages them. Are you testing using the mail or something else?

you know that with maxed out sneak there is no difference between light or heavy armor right?

The poison aura is active only when there's an hostile entity around (red dot visible on the compass). Unfortunately, when a enemy enters in search mode (no matter the distance), the aura will kick in immediately: if you happen to be real close to an enemy who still hasn't spotted you, the aura will damage him first thus blowing your cover. Companions and friendly NPCs are unaffected by the aura and I never received assault charges for having used the mail in group fights.

On the UESP.net page there's no mention to this, only that the combined weight of the apparel worn will influence the chances of sneaking. Note that I don't have taken the "shadow warrior" perk atm.

I don't really like the armor myself , but I tried it.Doesn't the description say something like "You are able to move more quietly?" If it's like every other thing I've found in this game then it's not actually mufle because it doesn't say muffle on it.Some opinions I've read say that the effect compensates for it being heavy or adding slightly to your sneak chance.But for me a piece of armor that gives off a random aoe effect (god knows what's gonna set it off) just seems rather goofy trying to play a serious stealth character.It does look really BA though =P my follower is wearing it.

Suggest trying the muffle spell and see if you notice an improvement in detection.Kind of curious now actually.

I like the look of it, better than the stock ebony plate mail (also, the texture seems to be of higher resolution).

Unfortunately my stealth character doesn't use magic so I don't have the muffle spell. But I do have the muffle perk which should have the same effect. It's useful while moving though i must say that sneaking naked is still the best option if you don't plan to enter combat at every possible chance!

According to the Elder Scrolls wikia, the shadow thing is not only visual and it will make your character 15-20% harder to detect.

It should be changed to light armor,or the skills fro mthieves should be able to work with it.You will lose alot of stealth benefits while using it.Or i'am wrong?Also i read about the Aura problems...i was thinking on using it.But i changed my mind.i'm going to use Nightgale set.Since it gaves me the bonus benefit's i want.

Well, as it has been stated: if you have a high sneak skill and muffled boots the difference between ebony mail and a light armor in detection is very shallow. In my experience.

I got a unique armor from a bandit named Linwe during a Thief guild quest. I'm now using the boots and bracers of this armor as their color closely match the color of the ebony mail. Although I lost the 'matched' bonus perk, the additional sneak bonus of this apparel, coupled with the bonus from the ebony armor create a good combination for a hybrid thief/sellsword character.

I also found a MOD which remove the 'black smoke' from the armor and improve the visiblity when using bows. Now I only need to find a helmet I like and whose color/shape can be matched to the rest of my outfit creating a pleasant effect