The term Watcher within the Necronicon seems to be used rather loosely in some places. It claims that Ningishzida is the watcher of the Gate to the Underworld. This to me seems to represent "guardian" or "gatekeeper" as that is her primary roll within the Decent of Inanna. It mentions the word Gatekeeper once when Ningishzida removes the jewel of Inanna, all other references seem to be that of "watcher".

It also mentions "watcher" in the 50 names a few times, once as the "watcher of the skies" (LUGALDURMAH) whom I believe simply "watches" the skies and does not really guard anything, and again with NARILUGGALDIMMERANKIA who watches the IGIGI and the ANNUNAKI. Again, I think this is more of a reference to "pays attention to" rather then guards. Only ASARULUDU within the Names deals with the Race of Watchers specifically. I'll speak of ASARULUDU a bit more later on.

Where the Watchers come from:
According to the "Normal Invocation of the Watcher" they come from the Underworld (Egurra & Under the Mountains of Kur both references to the 'land of no return'). Though, they are not native to that land. Also see below concerning their Parents.

The parents of the Watchers:
If you take the account of the Enuma Elish and compare it to the 50 names within the Nec it places ASARULUDU, NAMTILLAKU, and NAMRU as the gifts given to Marduk by Anshar, Lahmu, and Lahamu. This certainly places them above the race of man and apart from the Race of the Gods because the gods were not gods until Anshar begot Anu. As such the gods themselves should fear them as they out rank them by a generation.

The Watchers and the Covenant:
In The Testimony of the Mad Arab (part 1) it mentions the watchers and their Covenant between them, the Elder gods and the Race of men. In most other instances of the Covenant it simply says 'between the gods and man'. This leads me to believe that since they are apart from the Gods they abide by another set of Rules all together. Theoretically you can shoo away most Nec Entities in the name of the Covenant the Watchers will disregard that type of warning and kill you anyway. As further proof of a different covenant the watcher seems to be a Protector of sorts. Call upon another Entity to protect you and you must bind it in the name of a higher power, the watcher you must make sacrifice too and it does it of it's own accord.

What he looks like:
According to the The Conjuration of the Watcher he will come in one of three visages.
1) A great and fierce dog which prowls about what it guards.
2) A clean-shaven man wearing long robes and holding an unending stare.
3) At times he will come as the Enemy, but only to those that have erred, for they will be devoured by this silent race.

A possible Reference to the Watcher outside of the Necronomicon:

In "The Devils and evil spirits of Babylonia" by R. Campbell Thompson M.A. [1903] it has...
tug-šag-ga alad-šag-ga da-mu kan-gub
Is translated as
May a Kindly Spirit, a Kindly Guardian stand at my side
(literal spirit-heart spirit-heart side-name(ME) stand-assignment)

Thompson has translated it as "kindly" but more literally it is a Protector Spirit. The ancient text does not clarify one way or the other on if it is actually Nice or Mean. I've found similar references using Alad-sha-ga for a protective spirit as well.

So I am more then inclined to believe that this race is real and not just a figment of Simon's imagination.

Summoning the Watcher:
I will not go into detail about this as it's fully explained in the Nec. It is well advised that one read, understand and follow the instructions written within The Conjuration of the Watcher before one even thinks of summoning this entity!

Banishing the Watcher:
According to The Conjuration of the Watcher you should give him his license to depart by striking your left hand upon the hilt of the Sword and uttering a certain phrase. As the watcher is not of the race of the Gods and certainly not of the race of man I suspect that typical banishings will do NOTHING against them. I'd assume even the Banishing Crown of Anu would do little to stop this entity as in a way he out ranks the very gods one calls upon within that banishing.

As the Watcher can also be utilized to carry a curse to your victim, or worse to carry the curse of another priest unto you. It may be advisable to find a way to dispense of Watcher through other means, possibly through the aid of ASARULUDU. Though, as for how, I can not tell you.

Though, he does seem to be bound by certain laws concerning the his Covenant between the Gods and Man. So one may devise a system utilizing the ARRA and AGGA as they seem to hold some power of the Bandar... though I'm simply guessing here.

Humor:
I've left the Humor out of this as I do not think it's called for (and the watcher is NO laughing matter!), but if you are interested check the PSD homepage and look up possible translations for the word SSARATU. (TO FART)

Lastly, there seems to be a way to effect the watcher of another priest though I do not know how, and it's only speculation...but in The Testimony of the Mad Arab (Part 2) the Arab's Watcher seems to be afflicted with a disease. It does not go into any detail on how this could be possible so it may well be the Arab's powers is waning and effecting his control over the Watcher...but either way what causes this effect is thus far speculation.