Are just like normal hard drive's. They are great in my opinion for backing up onto and thing's like that, but for heavy use i would get worried about as i think they are slower than normal. This is because of the way that you access them. External's tend to use USB that can be slow, so unless you have a USB2 compatable hard drive, i would go for a normal drive, cheaper aswell!
If i had the choice between external and standard hard drive though i know what i would choose for keeping music and backing up onto, is the standard one everytime.