I recently had to move hosts and guests to a new vcenter server as the old server had become corrupt and full of issues.
The current vcenter has a few custom attributes and notes that would not be transferred as part of the move.
So I wanted to use powercli to read the attributes out and put them back.

To export the attributes I used the script below.
You will need to add as many Key Value pairs as you have custom attributes

#load Vmware Module
Add-PSSnapin VMware.VimAutomation.Core

Connect-VIServer -User 'VMUSER' -Password 'USerPasswd221' -Server 'vcenter1'

$vmlist = get-vm 
$Report =@()
    foreach ($vm in $vmlist) {
        $row = "" | Select Name, Notes, Key, Value, Key1, Value1, Key2, Value2, Key3, Value3
        $row.name = $vm.Name
        $row.Notes = $vm | select -ExpandProperty Notes
        $customattribs = $vm | select -ExpandProperty CustomFields
        $row.Key = $customattribs[0].Key
        $row.Value = $customattribs[0].value
        $row.Key1 = $customattribs[1].Key
        $row.Value1 = $customattribs[1].value
        $row.Key2 = $customattribs[2].Key
        $row.Value2 = $customattribs[2].value
        $row.Key3 = $customattribs[3].Key
        $row.Value3 = $customattribs[3].value    
        $Report += $row
    }

$report | Export-Csv "c:vms-with-notes-and-attributes.csv" -NoTypeInformation
									

It should produce a csv file that looks something like this

VMNAME,NOTES,CREATEDATE,CREATOR,DEPLOYDATE,TEAM
vmguest1,note1,12/29/2011,Bob,12/30/2011,Web
vmguest2,note2,12/29/2011,John,12/30/2011,Accounts
vmguest3,note3,12/29/2011,Paul,12/30/2011,Database
									

Once you have exported the file you need to import it into the new vCenter
again adding Key Value pairs as needed.

#load Vmware Module
Add-PSSnapin VMware.VimAutomation.Core

Connect-VIServer -User 'VMUSER' -Password 'USerPasswd221' -Server 'vcenter2'


$NewAttribs = Import-Csv "C:vms-with-notes-and-attributes.csv"

    foreach ($line in $NewAttribs) {
        set-vm -vm $line.Name -Description $line.Notes -Confirm:$false
        Set-CustomField -Entity (get-vm $line.Name) -Name $line.Key -Value $line.Value -confirm:$false
        Set-CustomField -Entity (get-vm $line.Name) -Name $line.Key1 -Value $line.Value1 -confirm:$false
        Set-CustomField -Entity (get-vm $line.Name) -Name $line.Key2 -Value $line.Value2 -confirm:$false
        Set-CustomField -Entity (get-vm $line.Name) -Name $line.Key3 -Value $line.Value3 -confirm:$false
    
    }

									

Hope this helps someone.