In the spirit of...both people to respond so far asking about it (in this thread, we shall start off with this elite, that has been much maligned since the original debuff, when people moved into Energy Drain and Offering of Blood and never looked back, but Arena.net has given this yet another buff, and one that almost makes me want to play it this time. So onward, to Peace and Harmony (for Peace and Harmony?)!
Peace and Harmony
Elite Enchantment Spell
Linked: Divine Favor
Available: Prophecies only; Marnta Doomspeaker (Snake Dance)
1 second cast
10 second recharge
For 30...78 seconds, target ally gains +1 energy regeneration. Peace and Harmony ends if that ally casts a spell that targets a foe or deals damage to a foe.
Peace and Harmony can be applied to multiple party members (so long as they do not try to harm enemies directly), which allows it to output some truly awesome energy.
Divine Favor-linked energy gain skills allow for greater power in healing/protection prayers.
It is an enchantment, and thus heavily strippable.
The careless will strip it from themselves.
Energy return is quite slow.
Of the elites, I believe this has the single greatest potential energy gain wrapped up in a single elite. Unfortunately, this maximum potential will never be achieved, as it's quite difficult to have a party that never targets enemies. The skill is mostly useless if applied to only one person, as is usually the case. It will generate one energy per three seconds, which means it takes a full fifteen seconds just to break even. At the listed maximum duration of 78 seconds, it has net the caster 21 energy. Compare this to, say, Energy Drain, which nets 7 energy per cast, with a twenty five second (but improvable) recast, with the added bonus of taking energy away from the opposition, even after it was debuffed, and it becomes apparent why was one of the weapons of choice in the past.
A subtler disadvantage of Peace and Harmony is that since it provides energy so slowly, it cannot be used as effectively to get out from under the thumb of one of those stepdancing fellows who refuse to show their faces. Energy Drain, Offering of Blood, and to a lesser extent, Mantra of Recall, which have been the traditional PvP "Energy Management" skills all provide energy in bursts, which the Mesmer has problems containing, at least for a short while. Simply gaining energy slightly faster over time is not always as effective (although it can be more so at times).
Back on the original topic, Peace and Harmony becomes much more efficient with each person it can be put onto, netting the same 21 energy over 78 seconds for each additional player, which can add up quite fast, assuming it stays put. The main problem here is making sure that happens.
But with a ten second recast, you say, there is no problem in re-applying it. This is partially true. Its main purpose of net energy gain is partially negated each time it is removed. If it stays on for ten seconds, you've actually lost energy in casting it. Fifteen will break even, and I doubt many PvP folks will let it stay up that long if it becomes apparent that you are relying on it.
There is also the potential disadvantage of not letting the Monks in question participate in spikes, as has been done in the past.
If we move to the PvE side of things, this has become an excellent skill for parties involving two to three monks and/or Ritualists, and I believe even the ever-popular Minion Master can keep this on himself, so long as he doesn't target enemies. Support Paragons may find it useful as well, though many of their skills become less efficient while they are enchanted.
PvE: An excellent skill IF one has the party for it. If not, leave it at home. I'd say you want at least three people who don't target or attack enemies to use this, as at least one will generally be stripped.
PvP: This is a bit more difficult to tell.
The Random Arena is worse than iffy, though it is the area with the least enchantment removal. There is very little chance of having another good target.
The Team Arena improves those chances, but even with a party of fortress ritualists, that's four people, and most of your opponents will have Shatters. You'd really have to build around the skill to make it work.
The Hall of Heroes, I'm afraid I can't comment on too much. Last I heard, though, it was littered with the mangled corpses of those relying on Enchantments for victory. Wells of the Profane fill nearly every corner, and all-stripping skills are used frequently in spike preparation.
Guild battles I can comment little more on, as I've not so much as set foot in observer mode in months, choosing to relegate what little online time I have to playing instead of watching. Last I saw, though, there was generally at least a Mesmer per group running at least two Shatters (or a penetrating Shatter). It won't stay on long, but in GvG, considering the battle is so spread out, it might be possible to make it work. At least you won't have to dodge Well of the Profane as much. This is also the PvP format with the most allies, and thus the most potential targets. I'll anything else here to those more informed than I.
(Feedback on the formatting would be appreciated. Too much history? More concrete examples? Leave it in the master thread with suggestion skills, linked above.)
And here's the obligatory disclaimer that I've no doubt forgotten something, and I'm either somewhat pressed for time or tired (Both today! What a treat!).