Unreliable and slow completely describes my experience with both Backblaze (lost the data for one of my drives which unfortunately I learned after the drive failed) and Crashplan (the Java client brings my rMBP to a crawl when it runs). There has got to be a decent online backup service that's reliable, reasonably priced, and respectful of resources out there somewhere.
Two that have been recommended to me recently that I haven't tried yet but would like to are Arq Backup [1] and Tarsnap [2].