If you have a modern, speedy system, you are certain to need several fans to cool everything properly. However, a fan farm can make quite a bit of noise. You don't need that cooling all the time - only when your computer is doing intensive work. I'll provide a brief overview of features you should be aware of, and will compare and contrast a couple fan controllers I have used.
If you are thinking about buying a fan controller, make sure you find the power specifications for it first. Controllers with inadequate power might just run your fans at a lower speed, or they might not run them at all. The Vantec Nexus is 18W per channel, and the Sunbeam Rheobus is 20W per channel - you should find ones that are similarly high, if you don't want to worry about it. If you can't find the specifications quickly, the company is probably trying to hide them, so be wary. The manufacturer's web site should have the specs, and if not you can go Googling. In addition to general reliability, a powerful controller allows you to run more than one fan from each channel. In my test bed, I have two fans each on the first three channels, and a more powerful, larger fan on the fourth channel.
You'll need to consider 3 mounting issues: 1) Most controllers fit nicely into a drive bay, but for some, it's a 5.25" bay, while for others it's an 3.5" bay. 2) The leads from the fans to your controller may not be sufficiently long if you have a very large case, or prefer to dress your wires neatly, in which case extender cables are available. Finally, you may well need adapters to connect your controllers to your fans. Controllers are available with all three-pin connectors, with all four-pin connectors, or with a mix. Make sure you make a list of the connectors your fans have before you buy - you may need several adaptors (generally cheap items).
Controllers often have lights, but their function varies. Some of them indicate different power levels, some of them are merely for decoration. I like the indicator lights, because they can in some cases tell you exactly what power level you are at. For example, the Sunbeam Rheobus has lights that are red up until 7V, then switch to blue at higher power levels.
Not all units allow you to turn a fan off completely, which is a big negative in my book. If you don't need a fan, it is much better to turn it off; even if it doesn't make much difference volume-wise, you will probably extend the life of your fans if you turn them off when they are unneeded.
More expensive fan controllers have temperature monitors and auto-adjusting fans, but I like to control mine manually, since I better know my cooling requirements, not just now but in the near future. If you don't know much about your system, though, it might be better to go that route so you don't have to worry about taking measurements or frying your system.
The units I looked at can be found at:
I recommend the Sunbeam Rheobus, as it can handle a larger power throughput and gives you more flexibility in control.