My entire school (approx 300 students) uses Dell Latitudes, and they are AWFUL. My class uses D600s. We discovered the batteries had been designed to fail a few months after the warranty expired (they would just stop working all of a sudden - battery life would be 3 hours one day, and nothing the next). Motherboards break constantly - buttons stop working, computers start randomly turning off, HD connections go bad, etc. The screen hinges are very fragile, as is the latch. The hard drives run so hot that they make it hard to use the laptop on your lap.
Worst of all is the buggy dell firmware. When we first got the computers, they couldn't be hibernated while the screen was set to its native resolution or the computer would never wake up. Even now, sleeping the computer can often result in problems when the computers wake. Wireless cards tend to be finicky as well, with poor software support.
Dells are great if you don't want to pay much and have free on-site service with free loaner laptops. Otherwise, I highly recommend going with IBMs. They are more solidly built and the IBM software is far better integrated. I have worked with both the D620s and the new Lenovos, and the Lenovos are far better. The D620 chassis feel like they are constructed from wet spaghetti.