They do get pretty warm, but cooling doesn't really affect them. However it gets the heat moving to where the cool air flow from your case fans can exhaust it more efficiently. You do have case exhaust fans, do you not? If not, forget the drive coolers and get some. A decent pair of fans, one for intake from the lower front, and one for exhaust at top rear can make a big difference in your cpu temps- 10-15 degrees! I improved 5 degrees (cpu temp) by changing to higher speed fans without any other changes.
Yes. 7200 and 10000 RPM drives can get really hot, although only 10K drives really need active cooling. Most 7200 RPM drives will not need auxiliary cooling as long as you have a well ventelated case, as the previous poster suggests. I would also refer you to http://www.storagereview.com for a look at what cooling is really necessary for today's HD's.
My workstation has an IBM 10K SCSI drive that gets extremely hot to the touch if it is not actively cooled, while most of the 7.5K drives are only warm...
I just take a 80mm case fan and remove the shroud with a pair of wire cutters. Then I slap a piece of 3m double stick tape on the fan motor and stick it to the flat side of the drive. Works awsome! And it's cheaper than cheap!