I had a few times to use both to restore some files and in one case my MBP HD that went bad... Through CrashPlan I was able to restore everything (it took almost 12 hours to re download everything) but at the end to my extreme surprise I lost less than 5 min of work since CrashPlan was quite up to date.
I do use of course Dropbox but I don't consider it as a backup destination and rather a sync'ing service. I do have for a few months now a FileTransporter from Connected Data and start to use it more and more since I can store up to 1TB and no monthly fee for it, but not yet done the full jump from Dropbox.
I will be curious to hear anyone else solution.
Time Machine for the Mac is excellent--I use it myself--but I must say that it is not sufficient by itself for the following reasons:
(1) Since the Time Machine backup drive is typically online all the time, it is vulnerable to malware that'll encrypt, corrupt, or erase you backups. (An example from the Windows world is CryptoLocker: http://en.wikipedia.org/wiki/Cryptolocker ).
(2) I imagine that most Time Machine users will have the Time Machine drive right next to their Mac. So whatever physical trauma befalls their Mac--fire, theft, electrical overvoltage--is going to affect their Time Machine drive as well.
Time Machine users need a real offline backup as well.
(I realize that you said that you use also use CrashPlan, a remote backup system, so my comment is not directed at you.)
It's comprehensive, covers off-line bare metal backups (which aren't exactly changing any time soon), points you at tools like rdiff-backup which you can use to get reasonably close to continuous data protection (I do it every hour), etc. etc. Along with a few good and short war stories. And preps you for the big times, if you're interested.
Making sure that your backups actually can be restored is also extremely important; there's not much worse than thinking that you have backups, but when you need them, find that they've become corrupted and unusable.
The only way to make sure that a backup lasts more than a couple of years is to copy it to new media every couple of years.
E.g. in the bad old days, not to mention to this day, I kept write enable rings on mag tapes (https://en.wikipedia.org/wiki/Write_protection) unless and until I had to do a restore, in which case they got removed before putting them in the drive.
I suspect a combination of optical media and parity data stored (PAR2 or PAR3 if I can find an open implementation) on flash media might be a good choice
You should have no problem finding an open implementation  
An other alternative is zfec , which has the advantage that you can choose the level of redundancy you want. It's also much faster.
zfec looks interesting though.