I use Gmail to collect mail from several other POP accounts, which works great. But you cannot control when Gmail chooses to check mail on those POP accounts. You can go into the settings and manually force each account to check, but that is inconvenient.
Thankfully, Google recently published a Labs feature to make this easier: http://gmailblog.blogspot.com/2010/03/new-in-labs-refresh-pop-accounts.html. If you enable this feature, the refresh link in Gmail will not only update the display, it will go check for mail on all of the POP accounts you have configured.
This is great when you know you have an email from another account and want to get it quickly. Thanks Google Labs!