Never used them myself, but have talked to several that have. The SPH-4 was a US mil helmet, now replaced with the HGU-56 with the US Army. Apparently it has better crashworthiness, it comes in more sizes (in theory better fitting), it has a thermoplastic liner which can be molded to fit the user's head better - it's also designed to support NVGs, and CEP which enhances audio feed while providing better hearing protection to the wearer. The guys I've spoken to seem to love them, though a few mentioned issues with dust clogging up the visor mechanism when in a sandy environment.
The Eagle seems to be more popular with civilian users, as does the Gallet - though in the past I understand Gallet had some issues with showing that their helmets provide an acceptable level of protection in impact tests. Maybe someone better informed can confirm or deny that.
Hope that helps. Like I said though, I've never used any of them, so I can't recommend any from personal experience