A web backup is better than nothing...but remember that you are completely at their mercy..besides charging outrageous prices for high volume of storage in the future, they can go out of business, or go offline when you need your backed up data. I don't see how this is a sustainable business model for the long run for these companies. I guess as long as lots of people don't back up lots of data and there is not lots of churn, there may be a profit margin in it. Its easy to collect money to backup data for one or two years, but where will they be in ten years?
With 1.5 Terrabyte external drives dropping below $129 I would try for redundancy at home, with offsite stoage of a hard disk/disk media in a safety deposit box for your most crucial data. Data is not 'going away' so its best to invest in managing it!