I use one everyday on my large hard drive. I haven't noticed a huge difference in its performance, probably none at all. I only have it on there because I had one laying around and I figured it was good insurance against premature wear and tear.
Honestly, they are pretty unnecessary for most if not all applications, but they're always nice for a bit of assurance that you are getting the most life out of your drives. Not to mention it keeps the air flowing around more in the system.
On the passive note, I believe Thermaltake makes one that is a heatpipe cooler, but it turnes the previously 3.5 inch drive into a 5.25 inch drive. Not to mention I believe its pretty expensive.
All in all, they're pretty inexpensive, but pretty useless in most cases. It won't usually make a lick of difference in performance, and while it may increase your MTBF, I have yet to actually prove that. That said, I personally would put one on at least the raptor, due to its high rotational speed, and probably one on the large one, just in case. It'd only be like 20 bucks or so for active ones. But if you want quiet(er), just leave it out and make sure plenty of space is between your drives.