From a purely noise viewpoint, it's much easier on the ears to have the voltage to the fans rise gradually when using an auto-control. From my experience, Speedfan is deficient in this respect. Speedfan waits too long to increase the voltage (as the temperature reaches the set point). Then right as the temperature starts to get too hot, Speedfan quickly rises the voltage. This quick rise in voltage/rpms......is very audible to me, much more so than a gradual voltage/rpm rise.
I've seen that complaint before but I can only assume it comes from people that don't know how to properly configure speedfan or people that got unlucky and got a motherboard that isn't speedfan friendly.
On the DFI nF4-DAGF motherboard speedfan will control through the full range from 0 to 100 percent. The fan I was most concerned with would stop at 15% and start at 20%. If you want it to move slowly to be less noticeable you need to go:
Configure - Options - Delta Value For Fan Speeds = x% - OK
I tried 5% and it was to fast. I tried 3% and it was reasonable, 2% was better, I finally set it at 1%.
I live in a heavily conditioned environment, small apartment with central heat and air. My ambient temp doesn't vary much at all.
On a Asus A8N-SLI Deluxe I only get control in a restricted range. I don't have the PC on so I don't remember the exact percentage but it is something like this:
0 to 35% give random RPMs in the top third of the fans full speed
35 to 40% give random rpms near the middle of the fans speed
41% to 100% give graduated control with some where in the 40 to 50% range having the lowest RPM of any possible choice from 0 to 100.
If you have the full range of response, a medium to high speed fan, and a steady ambient 1% delta is fine.
If you have a less controlled environment or insufficient airflow for your equipment even at 100% voltage a 2% or 3% delta might be needed.
The other big tweak to that is before you are sure about the settings and you are playing with 2 or 3% you should pay strong attention to how these settings affect the behavior:
Configure - Temperatures - X
and Configure - Fans/Speeds - X
(not sure on the second one there as I'm unfortunately on a PC that doesn't allow speedfan to control a fan.
The concepts here are:
Setting a min and max speed for each fan
Setting a mid and max temp for each temp monitored
Setting a relationship from each temp to none, one, or more than one fans.
On a full range motherboard your min speed might be 0% or it might be 1 to 5% above the stall speed. On the DAGF I tried 0% for a while but finally decided that setting the minimum to anything in the 17 to 20% range was fine, so I picked a min percent that gave me a round RPM number just for the fun of seeing the RPM I wanted.
On the Asus I set the min % to the actual lowest RPM I could achieve with speedfan changing the percentage 1% at a time and watching for the lowest RPM number.
Setting the high percentage is usually going to be a personal preference but if you bought cheap fans (which tend to have higher RPMs at 12v) you would choose a lower max percentage. If you have Nexus fans you might actually use 100% as your max.
In addition you should think of each fan as having 4 states with speedfan.
Max chosen RPM
The separators for those states are the two temps your choose and the max RPM you choose.
The "desired" temp is the threshold between Min RPM and Variable RPM
The "warning" temp is the threshold between Variable/max chosen RPM and 100% RPM
myself I buy cheap fans so the warning temp is an emergency option I never ever want to use. But for those people who buy fans such that the full 12v operation is still bearable the "warning" temp gives you much more flexibility on how complex the fan speed curve will be.
Depending on your usage pattern, choice of fans, settings, etc you might actually get in a situation where speed fan will move a fan up slowly without ever hitting the 3rd state (your chosen max RPMs).
Generally only the CPU fan needs a wide range of speeds.
Intake fans or fans specifically to cool a hard drive should be on all the time and shouldn't need much range in RPMs.
Exhaust fans can usually be fixed RPMs as the PSU fan is thermally controlled and in most cases is part of the exhaust flow.
If you only have 2 fan headers and you are measuring 5 temps with speedfan you will usually tell speedfan to tie fan RPMs to a single temp. The CPU fan header really doesn't need to be related to anything other than the CPU temp.
On a really good speedfan/motherboard combo you end up with 8 temp sensors with only a single core CPU and one hard drive and you can control 3 or more fans. The number of combinations of settings you can get going in speedfan is arbitrarily large.
It's not fire and forget and it isn't perfect but you can sure as heck get speedfan to do some wonderfully complicated fan control.