We never hear Darcy directly relay his opinion of his friend Bingley, but we may be able to infer a few things indirectly. For instance, based on his letter to Elizabeth we know that Darcy had "often seen him in love before" (Ch. 12, Vol. 2), so we can infer that Darcy has known Bingley for quite some time. Darcy is not the sort to remain friends with someone he does not respect and admire, therefore, we know that Darcy most certainly must respect and admire Bingley. We also know from Darcy's letter that Darcy cares a great deal for Bingley, so much so that he would not want to see his friend in a marriage in which his prospective in-laws acted with impropriety and lacked principles. Finally, we know from Darcy's letter that Darcy considers Bingley to be very modest and to value Darcy's opinion above his own.
When we first meet Darcy and Bingley, we hear Bingley declare that he "would not be as fastidious as [Darcy] for a kingdom!" (Ch. 3, Vo. 1). We may be able to assume that if Bingley sees such a drastic difference in their characters, Darcy may see it as well. Darcy may wish that Bingley is less accepting and less easily pleased than he is. Being a little more critical would give Bingley more discernment.