PotatoOoverodse is going to have an aneurysm when he reads this topic. And a hemorrhoid.
If you ask me the best use of this spell is to build a force of workers early, in advance, then cast it and keep them invisible for the rest of the game.
Monkeyfinger is quite right (especially about the hemorrhoid). This spell is one of the few things that makes a warrior rush still possible on higher difficulty levels as it allows you to use the AI's new found aggression against itself.
Here's the basic strategy:
1)Turn 1: Start building warriors.
2)Barbarians Attack, you get a few promoted defenders.
3)Turn 30-50: Nearest AI aggressor (there are plenty, hell I've had Einion and Cardith try to take me out on turn 30 in the same game once without aggressive AI enabled...) attacks. AI sends the majority of their units into your lands.
4)Cast Worldspell.
5)Leave well promoted defenders at home.
6)Send warriors on a happy saunter through enemy lands.
7)Discover that enemy cities are barely defended(they have a stack of "doom" on your doorstep).
8)Profit.
9)???
10)Eat Pie.
Actually, as of .40, hidden units can enter borders like HN units or Loki, meaning one could potentially bypass step 3 making the world spell significantly more useful for surprise attacks. Mobilize a huge army on the doorstep of their cities. Declare war. Profit.
Also, there is the classic invisibility+pillage, great for keeping neighboring AI economies in the gutter.
While I agree that invisibility/hidden aren't the most useful in general, this is largely because of the ridiculous effectiveness of hawks. Seeing as the Sidar world spell can be used before hawks, it does not suffer from this.