adagast was said to also have failed his mission, but it's tempting to
think that his "failure" was not as bad as that of the others. The Essay on
the Istari: "Indeed, of all the Istari, one only remained faithful, and he
was the last-comer. For Radagast, the fourth, became enamoured of the many
beasts and birds that dwelt in Middle-earth, and forsook Elves and Men, and
spent his days among the wild creatures." (UT, p. 390) Radagast certainly never became evil. The above quote suggests, however, that his mission was not just to relate to wild creatures but also to build bridges between them and Elves and Men. He did, in fact, have his friends the birds gather much information, but since they were reporting to Saruman as the head of the Council that wasn't altogether helpful. On the other hand, it has often been suggested (though there is no direct textual evidence of any kind) that the way Eagles kept showing up at opportune times may have been partially his work.
We know nothing of what happened to Radagast after the end of the Third Age. It seems conceivable, though, given the more ambiguous nature of his failing, that he might have been allowed back to Valinor eventually.