I don't see how they are immune to devaluation. Their 'value' is still dependent on a 'perceived' value.
There is no security in these systems, no insurance. If the system is hacked or broken, you are stuffed.
They are no more secure than cash or gold kept in a safe.
I was perhaps lazy in saying "immune to devaluation". They are immune to central banks devaluing them through Quantative Easing because there is known number of $coins which means they cannot simply be printed at will by the Bank of England. The value at which they trade is of course variable (and volatile!).
There is certainly no insurance against theft of crypto currency and as the security is being tested it's pretty clear that these are very early days for crypto currencies and I wouldn't be putting my pension into them. However, if you need to be pretty anonymous for online purchases of pornography, narcotics or you're financing an international terrorist ring - I would imagine they have their uses.
I'd just like to point out that my use of crypto currencies is limited to mining one Litecoin and then losing the password to my wallet - I've never bought anything with bitcoin