Using the Internet for backup is great because your backed-up data is physically separate from your Mac in case of theft or disaster. But given the slow throughput of the average modem-based Internet connection, Internet backup services are primarily useful for backing up a small set of important files, not your entire hard disk. When you're evaluating any Internet backup service, make sure your files will be compressed and encrypted.
Compression can reduce overall transmission time significantly.
Using encryption is the only way you can be sure that your data remains private as it travels across the Internet and is stored on remote servers. If you use Retrospect to back up a network of Macs, an Internet backup would be an excellent secondary, off-site backup of your most important files. If you had relatively few important files, you could rely entirely on Retrospect Express and Internet backups to avoid purchasing an expensive backup device. Retrospect treats an Internet FTP server as another type of media, just like a disk or tape. Since other people may have access to your FTP backups, be sure to turn on Retrospect's encryption when you create Internet backup sets.