Also, if the DC for your illusion is high enough, it won't matter if someone detects it as an illusion. All the rest of the party will get a saving throw because they were alerted to the possibility of it being illusion, but they might still believe in it. This can cause quite a bit of hilarity when half of a party believes the illusion while the other half disbelieves.
In fact, I'm not even certain that succeeding on the spellcraft check will allow you to bypass the will save to disbelieve anyway. It would certainly give you the opportunity to disbelieve, maybe with a circumstance bonus, but it won't garauntee. Maybe you think the illusion he just cast was cast somewhere else? Maybe you just had a momentary brain fart and neglected to put 2 and 2 together? Maybe you doubt your abilities? Maybe you know he just made an illusion, but that wall looks REALLY REALLY REAL and thus you must have been wrong, since that is obviously a real wall that was just put up?
Additional tactics: You are in a room full of holes. Make an illusion to make the holes look like the floor. Oh sure, you know that there're illusion floor panels... but, uh... did you memorize the EXACT location of all the holes? Just because you disbelieve doesn't make the illusion vanish. You still see it, you just know you can go through it.