I would say Google is the most stable. Though I dont see Microsoft going down anytime soon either.
Cloud backup is pretty safe, depending on what option you go with. http://www.carbonite.com/ is supposed to be a good one. Though I always prefer a local backup over online storage. Way too slow to upload anything over 30GB.
http://carbonite.custhelp.com/app/answers/detail/a_id/1440
The first 35GB of data can achieve upload speeds of up to 2 mbps (megabits per second). -- takes a bare minimum of 40 hours to do
Between 35GB - 200GB of data can have the upload speeds reach up to 512 kbps (kilobits per second). -- takes an aditional 750 hours to back up to 200 GiB (790 hours total)
200GB or more of data can be uploaded at up to 100 kbps (kilobits per second). -- will take another 6990 hours to back up 500 GiB (7780 hours total for 500 GiB)
So, in other words, to back up 500 GiB will take you A YEAR of CONTINUOUS backups due to Carbonite's evil throttling policy. Hence, why they are horrible. If you upload speed is above 2 mbit, you'll never see the advantage of it with Carbonite, and their service gets progressively worse with every upload. To put this in comparison, someone with a 5 mbit upload speed can upload 500 GiB in "only" 10 days with continuous uploading -- MUCH faster than what Carbonite offers (34 times faster, to be a bit more precise)
CrashPlan does not throttle and they allow you to "seed" your first upload: you mail them a seeded hard drive so that way your first upload doesn't take forever.