Run mine 24/7. No worrieth.
Problems I have had are to do with cheap barbs leaking, and crappy tubing going a weird brown-colour in the UV light, and setting like concrete which cracks when you try and bend it.
Also, cheap and nasty pumps make a hell of a racket, so leaving it on 24/7 might not be an option, as it'll keep you up 24/7!
I've had to replace my pump, tubing and coolant after 3 months. but with a bit of thought, it's worked well.
Performance-wise, don't expect an awful lot. Mine cooled the CPU to 7-10'C above ambient, but adding a Graphics card in the loop raised this to 14-16'C. this is with 10/8mm tubing and a 120.1 RAD with two fans push/pulling (btw- two fans on the RAD does increase the performance by a noticeable amount)
And as always, leak test! (though an hour should be fine, all these people who do it for 24 hours are loosing 23 hours valuable gaming time)
Good luck!
MB4