It is very good for the intestines if done properly and very bad for the intestines if not done properly.
There are some dangers you should know about.
Physical damage is a possibility if you push the enema nozzle into the rectal wall and cut it, or use way too much pressure, or use water that is too hot.
Another possible problem is from an electrolyte imbalance. If you use plain tap water, it will absorb your electrolytes out of your body leaving you weak, dizzy and washed out. If you have a heart problem that involves the electrical componant of your heart, it could be fatal.
Enemas are great and good for you but you should do it properly. There are many web sites that specialize in the subject. I suggest you check them out.