[Boromir88]

I'm going to have to go digging through the sources, but I think there are clear differences between Gandalf's success in the Istari mission, and the others' failures.

I wouldn't go so far as to say the 3 unknowns (Radagast, Alatar and Pallando) failed in the manner that Saruman did. None of them failed like Saruman who actually accepted Sauron's (his supposed Enemy) goals as the right path. Then again, did the 3 unknowns succeed in the way that Gandalf did? No. Still, I would separated their failing from Saruman's failing.

There are very late writings, which were unfinished and you can take from them what you wish. Like I said, I'll try to dig them up. Basically, in some very late writings Tolkien said that the Blue Wizards were vital in the resistance against Sauron in the East. They were sent in almost a special mission to go East and keep down the numbers of Men who would join Sauron's army. And they were indeed successful in this, and without the Blue Wizards efforts in the East, the armies of Gondor, Rohan, and the "resistance" led by Gandalf would have failed as Sauron's armies would have swelled and straight out over-whelmed them.

There are also some less than kind writings about Radagast, who basically became lazy. He was a "good" person, by aiding Gandalf a few times, but with regards to his mission became lazy. And in truth, Gandalf actually knew more about birds and nature than Radagast, which was supposed to be Radagast's "specialty."