Is RoundCube good for....

[Logicman]

Hello,
We are developing a web application which lets users upload files and keep track of their customer data. There have been numerous request by clients about having the ability to check their mail directly from within the web application.
Whats the best way to go about this? I was thinking of integrating RoundCube within the application and allowing the clients to access their imap accounts just like they do when the use Thunderbird or Outlook on their PC's. They will have external (remote) imap servers that will store the emails.

Is this a good option? Is there anything better? Since we will just provide them a webmail client (they still use their mail server to store and send emails) what performance bottleneck can I expect?
Any help on this issue will be greatly appreciated!