I've two of them in my new pc, I have to get my ear up to the case to actually hear them (and even then they are only just audable, oh and the PC is really rather quiet). I've also got a second 4yr old pc that uses seagates and they are just as quiet (though thats not much of a trick my second PC can sound like a helicopter taking off at times
, I'm never using 80mm fans again) Like Talon I've just replaced a first generation SATA drive with a newer model and that drive has been used 8 hours a day everyday for the past 3.5yrs and that's just an average day, at times it's been going for a good 24hrs on more than a couple of occasions (mainly when I've been rendering animations or doing video work and left going).
I've used seagates exclusively for the past 4 years and the noise they have generated is really rather minimal, I'd seriously doubt you'd ever notice them over the rest of the system. I dare say I'll be using seagates for the forseeable future.