If you're buying or building a server I'd definitely recommend rack mount stuff.
I'll probably be using a rack mount cases for all my computers from now on, regardless of whether they're actually servers or even going in a rack. They've got lots of room, easy to work in and swap out parts, giant fans, and a lot of times they've got handles on the front so you can easily haul them around. Not as pretty as high-end desktop cases, but for the cost the feature set is unbeatable, a $200 rackmount case will blow a $200 PC case out of the water every time. Also you can easily bolt, clamp, or mount them to furniture, vehicles, push carts, whatever.
The downside is that you may need to buy some extension cables for powering components, but considering that's like $5, I don't really think it's much of a downside. I guess they're not as pretty either, so if you're a lame person that likes stuffty cases with glass sides, ugly fans with LEDs, glow-in-the-dark RAM, and scratch-and-sniff power buttons or whatever then they're also not really good. Personally I prefer to have the computer hidden somewhere, under a desk, in a drawer, whatever, with just a keyboard, mouse, and monitor visible.
