I remember a thread about them while back and I got the impression the jury was out - as you say it works for some but not others.
I think it would depend on the puncture but I would be inclined to partially inflate the tyre (if possible) to create some resistance between tube and tyre so if any goo does escape it would (hopefully) create a seal, wait a bit and see if it is possible to up the tyre pressure (if the can of goo hasn't done that already).
Replacing the inner tube afterwards is a given - after running a set of Conti Top Contacts for 4 weeks and getting more punctures than I can remember I am happy to try anything that saves me repairing a puncture in the rain
It's all academic for me at the moment - it is in the panier just incase. I will be sure to report my findings if it comes to that!