Yaaaa!
I agree with Jeremy. In comparison, Fiendform is definately less powerful. Is the quasit the only form you can take? Or is it the most powerful form?
XP cost should be mandatory. I've never seen an XP cost that you could possibly get out of by making a Will save. Were that the case, my wizard would never take XP hits because Will just happens to be my best saving throw.

And my recent hit of 1000 XP for casting a Permanent Darkvision on myself would be negated. Yay! Free Darkvision!
Suggestions? If you seriously want to use the valiance, make it a higher level spell, say 8 or 9. It's a CR 10 creature. 10! That combined with the ability to cast your Sorcerer spells (I'm assuming that once you are in this form you can cast your other class spells as normal) is monumentous power at a relatively low level. Minimum level to cast 6th level spells is, what, 10? (I don't have my PHB :/) So you would then become a 10th level sorcerer and a 10th 'level' celestial. Yikes!
Also, the penalty needs to be increased significantly. I'm not exactly sure the detrimental effects of Fatigue, but I'm willing to bet that it hardly effects spell casters. Up the Fort DC to 15 like Cheiro suggested, and perhaps make the failed save render you immediately unconscious from tapping so much power? That way its power weighs as much as its danger. If you intend to use the form for battle, it is generally accepted as a bad idea to be unconscious throughout its duration.
10 minutes/level is kinda long. Does polymorph self last that long? What level is poly self? I think, if it's higher level than 6th, that this spell's duration be reduced to 1 minute/level and the casting time be reduced to 1 full round. If this spell is higher than poly self, then the duration
might be justified.
Just a few suggestions...looks pretty cool. Mellan would definately love this spell.