IMHO channel mixing is still the best way of colour->mono conversion. The Imgage->Adjustment->Desaturate command just reduces each channel to zero saturation, which I think is what Imager's method will do. Image->Mode->Grayscale weights each channel so that the three channels are combined in the proportions R:G:B 30%:59%:11%. This is supposed to produce an image which looks better to the eye.

LAB mode gives the same weighting as Grayscale but always looks lighter (so many people prefer it) as the L channel is deliberately darkened when combined with the A and B channels. Dan Margulis, the guru of LAB mode, doesn't recommend this method for mono conversion.

Which all leaves channel mixing as the best method, but it's also the most difficult one to get really good results - you need to analyse the channels carefully for best results.

Steve