I'm with K-Strider, my previous school was almost 100% NS Optimum machines and we had very few problems. a few PSU's every now and then but that was about it, they sent them and I fitted them so was only a day or two down time.

I've had problems with Dell in the past trying to get parts for machines, you're ok within the warranty but afterwards it's a nightmare.

I'm currently using my persuasive powers to get NSO on board here... we've currently got the dreaded Research Machines.. AGGGGHGHHHHHHH. it's not the machines so much as the software

good luck in what you decide.