Gaining

Has anyone actually used these? I see they look absolutely packed with calories. Would they be any better or worse than heavy cream? I also thought of mixing them with heavy cream to make the shake.

Is it a waste of money? Is it especially dangerous? does it work?

There are so many to choose from and I don't know where to start

They're frankly not really packed with calories. It's just maltodextrin or dextrose powder. Eating oils, fat, butter, cream are far more fattening.

If you want to go the powder route, any beer brewing store sells massive bags of dextrose or maltodextrin for pennies on the dollar. Do NOT spend the $40-50 for a little bin of it sold as "weight gainers" – they're a total ripoff.