If the army was too out of balance they wouldn't be releasing it.
That is not to say that there won't be some balance issues. The guys they have had play testing couldn't have though of every possibility, but it can't be too far off either.
It's definitly WAY to early to run and hide or sell your Eldar to buy Necrons. Let's wait and see what happens when these guys hit the table. I'm sure it's just a matter of time before the weaknesses are found and this forum is filled with wonderfully devious anti-Necron tacktics.
As to the Monolith: 14 = 12. I love bright lances
Unfortuantely, the Monolith has a living metal rule, which means no weapon gets its special abilites against it, so meltas are S8 1 dice pen, and brightlanes are just S8 I think.
Its only 235 points too, and harder than a land raider, i am SOOO gonna get the necrons!
And they have pariahs, whole units with phase weapons that ignore inv saves, good against the pesky seer council that pops up everywhere!
As far as weaknesses, They are horribly expensive and lack AP weapons is all I know, will have to wait and see the dex.