Yeah, I use them all the time for the specific memory layout part. At the same time, I've never had to go as far with optimization as trimming a few bytes off m

Yeah, I use them all the time for the specific memory layout part. At the same time, I've never had to go as far with optimization as trimming a few bytes off my structs for the program's memory footprint. If that struct is getting written to storage or shoved over the wire though, I'll always optimize/pack them down, both for the size reduction and because it makes it easy to reconstruct on any other CPU.