It's one of those things really where it's only worth it relative to what you're coming from, and your expectations.
If you're coming from regular hard disks then pretty much any SSD will seem like a revelation. If you're moving from an already decent SSD to the 840/840 Pro then it might end up being the case that the only difference you notice is in synthetic benchmarks.
Real world performance depends on what you actually do. If it's just games then you could be talking about a fraction of a second faster loading a map or something, whereas if you're transferring data around frequently as part of your work (e.g. audio processing, etc) or whatever then it could translate to several seconds on each transfer.
Ultimately at the bleeding edge you're always going to be paying considerably more for a few percent improvement. Whether this is something you can reconcile depends on your bank balance really