1TB for $10/mo is a big deal for me and I imagine that the 100GB for $2/mo plan is a big deal for the average user.
Can you upload it at usefully-high bandwidth?
Can you get that data back out when you want it?
CrashPlan, on the other hand, has a horrendous upload speed (a megabit per second or less at all times). The only reason I am using them at this point is because they don't delete your external drive data if you don't plug it back in. Backblaze removes your files after 30 days if the external drive isn't reconnected.
However, they are both backup services-- while you can access files from a web interface whenever you want, it's not the same as Dropbox or Google Drive. Plus, obviously, there's no syncing between computers.