If the PSU is a good quality but underpowered then your other components will not work, or may stop working when they start to draw a lot of power ( ie under load) but it is unlikely that anything would be damaged.
If the PSU is low quality or offers insufficient protection to the components then a PSU failure (whether it is underpowered or not) may damage other components.
It will only damage your components if anything happens to it like shorting out, If it doesnt have the proper protection it could take other things with it when it dies.
I ran my rig ( not my current one ) but a sli GTX 275 off a Generic 750W PSU for about 2 years and it ran fine. I bought a Powercooler 950 Modular PSU which blew up on me but luckily didnt take anything out.
I totally disagree with people saying Generic PSU's are totally unreliable but then would also say if you can afford it, get a decent one.
I work as a Technical advisor and deal with a lot of Samsung Products, Which are total crap. Even the best or most popular make can go wrong and cause problems.
Generic power supplies lead to all kinds of stability issues, delivering low or dirty power can cause other components to run at the bottom or even below the recommended specifications. The system may appear to work properly but it will usually have random problems that seem to appear and go away by magic at times. I can't even count the number of systems I fixed that appeared to work most of the time but a new power supply eliminated all the random problems.
Not.that true. Rosewill makes psus with 80+ & they are garbage. If the psu is bad the first signs are pc shuting off while gaming. My corsair tx750 did it until swapped it out. Had very little load on it too.