Not sure about eating the catkins but from various sources I have seen the following information:
If you boil the bark it is useful to bath swellings and inflammations and is good for sore inflammed throats.
Also similar to Oak bark, due to the tannin content, it can be used as a mild astringent.
The bark also produces the dye Aldine which in the past people used to chew the bark and use the spit to dye leather and other products.
But im tempted to try the catkins and see what they taste like. Although it wouldn't surprise me if they have a high tannin content too and end up dying your mouth if you eat lots.