Reliability: Generally speaking, data centers are backed up to the hilt. Some of the larger ones even back up to different geographical locations. Availability (hopefully 24/7) can also be important. Check for news of outages and the vendor’s own service blogs. If there are too many outages in service, buy accordingly. In truth, all the services we’re aware of are near-100-percent reliable.
An online backup service isn't much use if it doesn't make the process of restoring or recovering your data quick and simple. For example, a service should offer search tools for finding particular files in your backup. It's also desirable for a service to be able to replicate an entire folder-tree structure so that it can help you recover from bigger data losses. Keep in mind that if you buy a plan that covers just one computer, you may have to transfer the account to a new PC if you ever switch your main device or if you need to restore data from a damaged computer to a replacement.
SpiderOak's beauty is that it's a combination cloud syncing and storage service as well as a backup client all in one. Unlike some of the other services though, you'll have to pay for the storage you use. You get 2GB for free just for signing up, and you can get up to 10GB by referring friends. SpiderOak Plus nets you 100GB for $10/mo to use for syncing and backups, and every 100GB after that is another $10/mo. Plus, you can connect as many computers to any SpiderOak account as you want, so you're not paying by the system. You can read more about SpiderOak's plans here.