For what it's worth, I work on a facilities team maintaining a mega-church (we could fit 10,000 people on a sunday morning, so... It's big) and for most of our smaller work we use cordless Ryobi drills. The batteries seem to hold up for a good long time and the drills are quite decent for the money.
But yeah, a corded drill would always be preferable for larger tasks. Cordless stuff is designed to be weaker (eg: slower speed, weaker motor, etc) so the batteries last longer. Corded equipment has unlimited power, so they can build them as strong as they need. For small tasks, it isn't a problem, but if you have to, say, drill lots of holes in metal or concrete, you definitely want a corded drill.