[Boromir88]

Originally Posted by Legate

And lack of access to the archives - that is ridiculous. Saruman (or in fact, all the Istari) was supposed to help the folks of Middle-Earth, among other things, with their knowledge. A Steward should then come and say "hey, you have no permission to look into my archives"? That'd be pretty stupid, wouldn't it?

And according to Gandalf Denethor was more welcoming to Saruman perusing the Minas Tirith archives, than when Gandalf goes to Minas Tirith and reads Isildur's scroll:

'With that thought, I forsook the chase, and passed swifty to Gondor. In former days the members of my order had been well received there, and Saruman most of all. Often he had been for long the guest of the Lords of the City. Less welcome did the Lord Denethor show me then than of old, and grudgingly he permitted me to search among his hoarded scrolls and books.'~The Council of Elrond

If anything, Saruman was the Istari member who spent a lot of time in Minas Tirith's archives and as a welcomed guest of the Stewards.

The two Blue Wizards are perhaps the trickiest of the bunch to reach any conclusions as. As far as Radagast, I agree with Legate his apathy and laziness to completing his mission of rallying suppport to defeat Sauron meant he failed, however he had never "fallen" to evil. Just as there is evil that exists in Middle-earth, completely independent of Sauron, with a strong power of its own (think Old Man Willow), there are overall good forces yet rather apathetic towards the fight against Sauron.

The Blue Wizards are slightly different, because it seems as if their fate is unknown. They travel East, with Saruman (but do not return), and perhaps start cults of their own, or perhaps just die and have very little impact on the events of the Third Age, primarily dealing with Saruman. But in his late writings, Tolkien seems to revise and imply Alatar and Pallando have a huge effect on the Third Age, curtailing Sauron's influence in the East. That Alatar and Pallando were sent specifically to the East (lending credence to TheLostPilgrim's post of each Istari having separate missions) and their actions in the East swayed many away from Sauron who's forces would have simply overwhelmed the West, had they not been sent:

But the other two Istari were sent for a different purpose. Morinehtar and Romestamo. Darkness-slayer and East-helper. Their task was to circumvent Sauron: to bring help to the few tribes of Men that had rebelled from Melkor-worship, to stir -up rebellion...and after his first fall to search out his hiding (in which they failed) and to cause dissension and disarray among the dark East. They must have had very great influence on the history of the Second Age and Third Age in weakening and disarrayinbg the forces of the East...who would both in the Second and Third Age otherwise have outnumbered the West.~HOME 12: Last Writings

The issue with this is it directly contradicts the published Lord of The Rings which states that all the Istari arrived in Middle-earth at the same time, and here Tolkien is writing the Blue Wizards impacted both the 2nd and 3rd ages.

However, it should still illustrated the point that, I believed all the Istari were sent with sort of a specialized mission, meant to contribute to the overall purpose of defeating Sauron. They were not meant to directly use their power to fight Sauron's power, but instead to use the powers in the people and creatures of Middle-earth to defeat Sauron. Gandalf clearly succeeds in this, with his constant weary fight in aiding Elves, Men, Hobbits, and dwarves against Sauron. Saruman was meant to do much the same, and perhaps use his knowledge of craft and voice to help. Radagast seems most likely meant to get the animals, creatures, and nature united against Sauron. While Alatar and Pallando were specifically sent East to do what they can in halting Sauron's influence in places like Rhun, Harad, and Khand.