I found this script to be the most effective for creating MySites in advance for all users.

[ps]
#———————————————————————————
#The sample scripts are not supported under any Microsoft standard support
#program or service. The sample scripts are provided AS IS without warranty
#of any kind. Microsoft further disclaims all implied warranties including,
#without limitation, any implied warranties of merchantability or of fitness for
#a particular purpose. The entire risk arising out of the use or performance of
#the sample scripts and documentation remains with you. In no event shall
#Microsoft, its authors, or anyone else involved in the creation, production, or
#delivery of the scripts be liable for any damages whatsoever (including,
#without limitation, damages for loss of business profits, business interruption,
#loss of business information, or other pecuniary loss) arising out of the use
#of or inability to use the sample scripts or documentation, even if Microsoft
#has been advised of the possibility of such damages
#———————————————————————————

Function New-OSCPersonlSite
{
<#
.SYNOPSIS
New-OSCPersonlSite is an advanced function which can be used to create personal site for each user in a SharePoint site.
.DESCRIPTION
New-OSCPersonlSite is an advanced function which can be used to create personal site for each user in a SharePoint site.
.PARAMETER SiteUrl
The specified site URL.
.EXAMPLE
C:\PS> New-OSCPersonlSite -SiteURL "http://sp2010:8888/sites/TopSite2"

This command shows how to create personal site for each user in site "http://sp2010:8888/sites/TopSite2".
#>
[CmdletBinding()]
Param
(
[Parameter(Mandatory = $True,Position=0)]
[String]$SiteURL
)
#Add "Microsoft.SharePoint.PowerShell" Snapin
if ((Get-PSSnapin "Microsoft.SharePoint.PowerShell" -ErrorAction SilentlyContinue) -eq $null)
{
Add-PSSnapin "Microsoft.SharePoint.PowerShell"
}
#Load "Microsoft.Office.Server" Assembly
[Reflection.Assembly]::LoadWithPartialName("Microsoft.Office.Server") | Out-Null
#Get SharePoint site
$Site = Get-SPSite -Identity $SiteURL
#Get service context
$context = Get-SPServiceContext -Site $site
$upm = New-Object Microsoft.Office.Server.UserProfiles.UserProfileManager($context)
$AllProfiles = $upm.GetEnumerator()
#Create personal site for each user
foreach($profile in $AllProfiles)
{
$AccountName = $profile[[Microsoft.Office.Server.UserProfiles.PropertyConstants]::AccountName].Value
Try
{
if($profile.PersonalSite -eq $Null)
{
write-host "Creating personel site for $AccountName"
$profile.CreatePersonalSite()
write-host "Personal Site Admin has assigned"
}
else
{
Write-Warning "$AccountName already has personel site"
}
}
Catch
{
Write-Error "Failed to create personal site for ‘$AccountName’"
}
}
$Site.Dispose();
}
New-OSCPersonlSite -SiteURL "http://intranet/mysites/profile"

[/ps]

Source: http://gallery.technet.microsoft.com/office/Pre-Provision-SharePoint-ec781ee9