Code becomes concise and elegant using the -pipelinevariable parameter, 

$report = Get-Datacenter -pv datacenter | Get-Cluster -pv cluster | Get-VM -pv vm | Get-HardDisk -pv harddisk | ForEach-Object {
     [psCustomObject] @{
        VM = $vm.name
        Datacenter = $datacenter.Name
        Cluster = $cluster.Name
        HardDisk = $harddisk.Name
        Datastore = $HardDisk.FileName.Split("]")[0].TrimStart("[")
        SizeGB = $harddisk.CapacityGB
        DiskFormat = $harddisk.StorageFormat
        DiskType = $harddisk.DiskType
        VMDKpath = $harddisk.FileName
     }
}
$report | Out-GridView

Cheers
Xavi