These sorts of services are rate-limited based on your upload bandwidth, which is why a lot of them include serious de-duplication methodologies to reduce how much data has to be beamed back to the mother-cloud. Whether or not they can do block-level or file-level de-dup varies based on the backup vendor, and may or may not include agents you have to install on your hosts. App-specific backups (SharePoint, Exchange, other databases) may not allow de-dup at all. In many cases, the initial copy-everything-to-the-cloud backup is by far the biggest, some vendors actually ship you a NAS-device in the mail for this step, but everything after that is effectively an incremental based on the previous backup.
The reason there aren't many concrete, "This is how it is, slim," style reviews is because there are so many variables affecting performance of these systems that it is hard to control for them. What are the variables?
- Network speed between your data and the cloud.
- How much data you need coverage for.
- Your data-change rate between backup periods.
- What kinds of data you need backed up.
- The compressibility of your data.
- The de-duplication technology used by the online backup vendor.
- Agent support for your apps.
Some of these are prime fodder for reviews (agents, de-dup methods, cloud network speeds) but the rest is highly dependent on your exact environment. Agent support is a biggie, since data environments are complex. You may end up with a combination of online backup and local backup as a result.
You just have to do your own fit-analysis.