I use them. I try to vary the text from one image to the next and to make the alt text relevant to the image while still including important search terms. Just don't make them all the same...
I simply use a short description of the image, but make sure I add a few keywords. Early on I got some good results from google images SERPs.
This is a good place to "rephrase" things. If you are using keyword phrases instead of single keywords, this is a good place to swap the phrase around a bit as well as mix and match. IMHO. As stated earlier by minstrel, just don't use the same over and over again.
There is no magic number or percentage, web-rover. There are a lot of articles out there about "keyword density" and they are pretty much bunk or out of date. Best advice is make it look natural to the image and make it so when a human visitor hovers over the image and the alt text pops up that visitor doesn't go, "ewww... spammy!" -- make it look natural and no SE will object to it.