Online backup has come a long way in the past couple of years. Service has become more reliable and faster Internet connections have made backing up large quantities of data more palatable. My Internet connection is 5Mbps (30 MB/min) upstream, which is more than fast enough for overnight backup.
In spite of these advances, my opinion is that online backup is still best suited for last-ditch disaster recovery. Any online backup strategy should be paired with a primary backup strategy that includes an image and/or file backup to a USB drive or network drive. Buying 100-GB of online storage is quite expensive, much less 1-TB. The "unlimited" plans are attractive, but companies make significant compromises to make these unlimited plans affordable, including lower backup speeds, lack of geographic redundancy, and subpar technical support. My research indicates that cheap and reliable are mutually exclusive for online backup - the lower the price, the less reliable the service.
It should also be noted that it's the recovery that's most important, not the backup. Numerous people reported that Mozy could take several days (or more!) to prepare to restore a large number of files. Ordering a DVD was even worse. For me, this isn't acceptable.
Here's my experience with the various services.
Acronis. My first attempt at online backup was with Acronis, who introduced online backup in October 2009. My experience with it has been total failure. Acronis TrueImage 11 has never succeeded in creating a backup on my Windows Vista system. Acronis recently increased my backup space from 25GB to 250GB, all for $50/year. This seems like a safe choice for them, since I've never been able to backup more than 5GB to their servers.
Mozy I've been using Mozy Pro for about two months now. They charge 50 cents/GB/month for their Pro version. On the plus side, the nightly backup works quite reliably and I was easily able to select what I wanted to back up.
However, there are several downsides to Mozy.
Second, numerous people have reported problems restoring data from Mozy, even from their Pro service. Former employees of Mozy confirm that Mozy has been having problems managing rapid growth.
Carbonite I didn't try Carbonite, although I know several people who are happy with it.
Iron Mountain I will be trying Iron Mountain next. They are the most expensive, but, compared to the cost of losing the data, the price is cheap. They support multiple historical revisions, incremental block updates, Windows 7, and many other features. Iron Mountain makes backup systems for large companies, so I'm hopeful that their technology is more reliable. I'll keep you posted.
To recap, although online backup has been around for several years, most of the solutions still have significant shortcomings, especially with reliability. My research is that you get what you pay for, with no exceptions.