I think Bingley & Darcy's friendship is genuine - there is definite affection on both sides. Bingley might be seen as weak, I suppose, but as you say, he is really open and friendly. Darcy has never struck me as the type to enjoy the company of 'yes men' - certainly, Caroline's constant flattery and fawning does not incline his heart her way! - so I can only assume that the friendship between him and Bingley is a case of opposites (in terms of temper) attracting.