On January 6, 1839 Robert Schumann wrote from Vienna to the publishing firm of Breitkopf & Hartel:
Several days ago I visited Franz Schubert's brother and saw with astonishment the treasures that are in his keeping. There are . . . four or five symphonies ... You could bring out the symphonies in arrangements for pianoforte-duet, a work that I would gladly undertake myself. There would be only a moderate fee. Schubert's brother, however, cannot forego a fee entirely, since he is quite without means, a father of eight children, and the posthumous works constitute all his property.
In another correspondence Schumann described his meeting with Ferdinand Schubert:
He [Ferdinand] knew of me because of that veneration for his brother which I have so often publicly expressed; told me and showed me many things. . . . Finally, he allowed me to see those treasured compositions of Schubert's which he still possesses. The sight of this hoard of riches thrilled me with joy; where to begin, where to end! Among other things, he drew my attention to the scores of several symphonies, many of which have never as yet been heard, but were shelved as too heavy and turgid.
On December 11 1839, Schumann wrote to his fiancée:
Clara, today I was transported. A symphony of Franz Schubert's was played at the rehearsal. If only you had been there. I cannot describe it to you; the instruments, they are human voices, and spirited beyond measure, and this instrumentation in spite of Beethoven - and this length, this heavenly length like a novel in four volumes, longer than the Ninth Symphony. I was completely happy, and wished for nothing else save that you might be my wife and I could also write such symphonies.
Robert Schumann's posthumous reputation as a composer has eclipsed his role as indefatiguable champion of worthwhile music, but by launching Franz Schubert's Symphony No. 9 in C Major he bequeathed a precious treasure to subsequent generations. The symphony, performed during Schubert's lifetime only in a desultory reading in Vienna, had been moldering in Ferdinand's care, just waiting to see the light of day.
Schumann's friend and colleague Felix Mendelssohn also recognized the work's worth. He wrote to that fine musician Ignaz Moscheles on April 4, 1839 that:
At the last concert we gave a most highly significant and interesting symphony by Fr. Schubert; it is, in any case, one of the best of the newer pieces that we have; alive, piquant, and characteristic throughout, and it stands foremost amongst Schubert's instrumental pieces.
Notwithstanding such powerful championship, many listeners and performers required more convincing. The C Major Symphony's immense length worked against it, even when trimmed down as was typical of those early concerts. An attempt to perform the work in London foundered on the rocks of orchestra resistance; even as late as 1856 it was being performed in snippets rather than in one long go.
Those times are long past, fortunately, and nowadays the "Great" C Major Symphony enjoys repertory security, generally protected from ill-advised trimming or re-working.

The characters in the drama: Franz and Ferdinand Schubert, Robert Schumann, and Felix Mendelssohn
Schubert's apparent simplicity and lyrical sweep mask an underlying harmonic richness. A perfect example can be found in the first movement, just as the shift from the tonic key (C Major) to the secondary key of G Major takes place, after a breathtaking foreshadowing of the secondary theme in E Minor. Once that arrival in G Major takes place, a series of bold modulations helps to prolong the inevitable closure -- from G Major to B Minor to an E-flat Major which itself is prolonged via neighboring A-flat minor chords, the whole finally resolving properly back into G Major. (For those of you with some harmonic background: an E-flat Major triad is easily refitted as an augmented sixth chord that resolves to the dominant of G Major and hence quickly home.)
But harmonic fascination aside, the work's grandeur, its joy, its lyrical effusion, and its oh-so-relaxed attitude towards getting to the point, render it endlessly fascinating and deservedly well-loved for generations.
As you might expect, the Schubert C Major Symphony has been extremely well served on recordings; ArkivMusic lists 106 albums available. I wouldn't pretend to be familiar with more than a sliver of those, but I do have a few favorites. All links are to ArkivMusic.com, but you can find most of these elsewhere, also in download format from the usual vendors (iTunes Store, ClassicsOnline, Amazon, TheClassicalShop, DG, etc.)
Michael Tilson Thomas conducts the San Francisco Symphony this coming week in performances of Schubert's C Major Symphony, together with Alban Berg's Chamber Concerto with violinist Julia Fischer.
The performances are:
Here's the concert page on the SFS website, complete with ticket sales and other such fine things.