It was my son's turn to get the new case, so I mounted up his microATX board, video card and wireless adapter. I'm using a Glacialtech tower style cooler similar to the Thermalright Ultra-90 in size, which I mention now as it will come into play later when the side panel is reinstalled. With a single DVD, floppy and SATA hard drive there is plenty of room for additional components. The tool-less riser clips and drive locks all work flawlessly, as do the hard drive rails. I found some of the cables to be overly long, however I suppose that's always preferable to being too short.
I did have a bit of an issue with the cables coming from the front I/O ports, as they stretch right across the 3½" openings where I tried to mount a floppy drive. I had to fiddle with it a few times to get them to route either above or below the drive without getting pinched when reinstalling the front panel. And as I alluded to previously, there isn't a whole lot of clearance between the top of my tower cooler and the side panel fan, only about half an inch. A little larger CPU cooler and you'll have to remove the fan to get the door on.
There are three probes in the Guardian 921, one more than many cases that offer a built-in temperature display. I affixed the HDD probe to the top of the hard drive with a small piece of tape, and stuffed the CPU probe between the fins of the heatsink. I opted to slide the SYS probe between the two memory modules to monitor RAM temperature instead. The LCD display has large numbers and is easy to read, although overall it's still small compared to the size of the housing in the panel. NZXT could have easily made this smaller, in fact it wouldn't need to stick up above the top of the case at all. The LEDs are very bright, although not too much light comes through the vents in the front.
On to final thoughts and conclusion.