One of the good things and bad things about Android is all the choices for applications. It comes with its own mail and Gmail apps, which are good. I like to have all my accounts in one. I looked and tried a variety, but finally settled on K-9 Mail (http://code.google.com/p/k9mail/). An open source mail client, K-9 allows you to add all your accounts and either view them separately or within a Unified Inbox. I really like this app, as I can easily switch from one account to another within the same application.
In using the Unified Inbox, I really like the ability to color-code my accounts to help with determining which is which. On their site, the folks at K-9 discuss how they base it on the stock mail app, but in using it, they definitely improved the mail. If you utilize multiple accounts, K-9 will definitely help you out–check it out!