No worse than hiking Half Dome, less crowded, and it's a better view--you can see Half Dome from El Cap. I never hiked it as a standalone day hike, just as part of a backpack on the North Rim.
If you go to El Cap, take the time and look over the edge. Last time I did it, two woman climbers came up and over the rim. Be very careful though, people have fallen from the rim.
Also consider starting your hike to El Capitan from Tamarack Flat Campground, see this thread:
viewtopic/t=279Another possibility is starting from Yosemite Creek Campground and heading back.
Any way you do it, it's a long day--I would start around sunrise, if not before and bring plenty of water. Good luck.