Adding some qualitative experience with a few of these fans...
I recently got my hands on a few of these fans at cost from a friend who works at an internet retailer. I tried the S-Flex SFF21F 1600rpm fan, the 1200rpm Slipstream, and the Zalman ZM-F3.
I run an overclocked q6600 on a Xigmatek HDT-S1283, and had read countless fan comparisons on xtremesystems forums, silentpc forums, etc.
I run my system in an Antec 300 case, which has a 140mm fan on top, easily the loudest fan in my case. It's not particularly bothersome, as it's pretty low pitched, and it sits right above the cpu heatsink, so while it drowns out the cpu fan a bit, there's large openings on the top, back, and a hole in the side of the case for you to hear the cpu fan. For this reason I am somewhat picky about noise, as I sleep pretty close to my computer.
I have had other versions of the slipstreams, a 1600rpm one and an 800rpm one I still use. Both I have tried on the heatsink, the 1600rpm one was too loud for me, even when brought down to 1250rpm. I hoped this would change with the 1200rpm one.
It didn't, really. The 1200rpm one sounds great free air, only a light low pitched hum. When put on the heatsink, the sound is not particularly loud, but is a noise that stands out from the other fan noise, low and prominent. I couldn't stand this noise, it drove me crazy and I couldn't use it. But it wasn't all that loud.
Next I tried the Zalman, and even at 1800rpm, I thought it sounded better than the Slipstream. Louder, but it blended in better with my other fans and wasn't too distracting. Performance was the best with this fan, both idle and load, load temps could best the Slipstream by 2-4 degrees, not quite sure because of different ambients. But significantly better. Idle temps were better too, by 1-2 degrees. The only problem I had with this fan was when I throttled it down, I would hear a light ticking noise. You wouldn't have known it was there if you didn't put your ear close to the fan, but after you know it's there you don't forget about it and start noticing it more. It could have been just my sample, but I was disappointed as this was a great performing fan which seemed perfect for automated fan control, it was very quiet at lower rpm's except for the light ticking.
Lastly, I tried out the S-Flex. At 1600 rpm, it is more quiet to me than either the Slipstream or the Zalman, and definitely has the best tone to it. It just doesn't bother you. The performance is on par with the 1200rpm Slipstream, maybe just a bit better, tough to say with ambient differences. It scales down perfectly, I would recommend this fan to anyone with a Xiggy or similar. This fan has a bit of internet "hype" if that's possible for a computer fan, and it lives up to its billing. *edit* After using this fan a little longer I think it's a better performer than the slipstream and on par with the Zalman, I equaled the Zalman's best temps when it cooled down at night. Still very little difference between these three fans, we're talking couple degrees here or there, nothing life changing.
All this said, I realize it's tough to decide what's worth the money to you, the differences are just tones and the performance leaves me with a safe overclock regardless of which one I choose. I was lucky enough to get these fans at cost, $5 for the Slipstream and $9 each for the Zalman and the S-Flex. Currently I'm running the S-Flex on the Xiggy and the 1200 Slipstream on the exhaust. It's a great combo and it doesn't bother my sleep, so I'm quite satisfied.
