I thought that SSDs, along with all flash memory, are unreliable?
They are limited in some respects, and much more expensive per gibibyte, but they don't share the same weaknesses as regular magnetic drives, so they are an acceptable medium for backups.
The life expectancy for most current SSDs is much longer than traditional hard drives.
Depends on what you are doing. Their life expectancy is neither necessarily longer nor necessarily shorter. This is why they are an acceptable medium for backup, as they are just plain different.
It might be a bit expensive, but it's extremely convenient.
I backup my laptop hard drive every 2 weeks to an external drive, but I use 100GB dropbox account for work related files and it is always synced on desktop/laptop/work computer/iphone without thinking about it.
I went through a few different laptops during 2010 and it was convenient to have everything on dropbox.
Like I said, Dropbox is great for syncing, but absolutely horrible for online backups. Don't get me wrong, I love Dropbox for what it is, but let's take a look at the paid-service alternatives:
Remember: Dropbox is $200/yr for 100 GiBs
https://www.sugarsync.com/products/sync_pricing.html: $150/yr for 100 GiB, but we can still do better. Not to mention it isn't limited to a single folder like Dropbox
https://spideroak.com/pricing: $100/yr for 100 GiB, once again it isn't limited to a single folder like Dropbox, but I think we can still do better.
http://www.wuala.com/en/pricing/: $130/yr for 100 GiB, BUT you can trade your local storage for free backup storage, meaning it could potentially be almost free. However, we still might just barely be able to do better
http://explore.live.com/windows-live-mesh-sync-p2p-using?os=other: Completely free for up to potentially 400 GiB, but it is completely Peer-to-Peer syncing, you don't get the cloud aspect.
Not to mention for just cloud-based backups the standard price is between $50-75 unlimited, significantly less than Dropbox, and CrashPlan can potentially do it unlimited for free.
Edit: two more services:
http://www.livedrive.com/Promotion/mozy - $128/yr through their promotional offer for unlimited backup and syncing.
http://us.trendmicro.com/us/products/personal/safe-sync/index.html - $60/yr for unlimited syncing and backup
So in case you haven't gotten it yet: Dropbox just doesn't offer competitive prices at all, but I still love their free service