They are not a hay/pasture replacer but are quite nice to add fiber and calories to horse's feed. As for how much you should feed, this is a difficult question as it seems everyone has a different opinion and it obviously depends on the horse.
You need to soak them ideally as they swell 10x their size ( like speedibeet) I feed one Stubbs scoop of soaked sloppy grass nuts in each feed over winter to my tbx, slightly poor doer, less now the grass has started to come through though.
I noticed a difference when I switched from unmollassed speedibeet onto soaked grassnuts.
They're cheap as well, I feed northern crop driers grass nuts and they're £7.50 a bag. They're basically fresh cut dried and compressed grass. They smell delicious, really sweet and grassy(!) - they do have a higher sugar content than some would like, however my horse is barefoot and copes well with them. I really rate them, especially for those on poor grazing, you're just replacing what should be naturally available to them.
I'm another that things they're fab. Brilliant for adding condition. They also seem to provide controllable energy for my TBs.
as said before soak well - doesn't have to be overnight, an hour or so can break them down enough to make them feedable.
Thank you for your replies. I now have a bag of grass nuts but have yet to introduce these to his diet. I think he will like them, as he is not fussy and will eat any food!