immediate actions

[bjoern]

An immediate action can interrupt another creatures turn. That's pretty much all it says , at least as far as I can find.
Are there any limitations to this?
Do you have to wait until the end of the current action to do it?

A guy charges you, when he's 10' away(still in the middle of his charge), you interrupt with an immediate action and cast celerity.

This seems pretty overpowered to me, but I haven't seen anything that says it doesn't work this way.

[heavyfuel]

Yes it works that way, yes it can be op as hell. [see also: Abrupt Jaunt]

[Fax Celestis]

Immediate actions are instant, can interrupt other actions, and do not happen before the triggering action (if any), unlike attacks of opportunity. That's why they're immediate.

[nedz]

Only 1 per round and they eat your swift action for your next turn — so no Quicken etc.

[Curmudgeon]

Are there any limitations to this?
Here's a big one:
You also cannot use an immediate action if you are flat-footed.

[bjoern]

Does it use up the swift action for next turn?
Or are you unable to takean immediate action if you took a swift action on you last turn?

[Fax Celestis]

It eats your next swift action if used on someone else's turn, or your current swift if used on your turn.