Yes, it's free of charge if your data plan allows unlimited data traffic.

We never charge you for using Yahoo! Go 3.0.

Your network carrier does charge for data traffic in some way, depending on the data plan you have. Unfortunately, there is no way that Yahoo! can tell you about the charges your network carrier might put on downloading, installing, and using Yahoo! Go 3.0. You would need to go to your network carrier and ask them how much is charged for data and how much data traffic you are allowed. To help you determine how much your network carrier charged you for a session, Yahoo! Go 3.0 shows how much data was used for that session when you exit.

As a general rule, keep in mind that large data items are photos and mail attachments. If you are using very many of these, you should find out how your network carrier accounts for the large data traffic generated by those items.