I also bought a SFF for no specific reason (Biostar IDEQ 200vb) and love the thing! Noise is definately not a disadvantage with this one seeing as it's the quietest computer i've ever owned. I started out building with gigantic server cases, moved to mid-towers - now these SFF units. I don't know if it's a phase, but they do look pretty sleek next to my Dell 1800FP LCD.
I do have to say that I wish they had a little bit more juice than the 200-250watt power supplies they normally come with. I have a feeling that my 9800Pro that I have in there is a little picky about it at times. It's only a matter of time until they start selling these things so you can tweak the BIOS a bit, not much tweaking in general IMO.
As for room to move around inside for installing, ect. - not much to fiddle with considering most have only 1 agp and a couple of PCI slots at most. I use to deck out all my machines with lights, heatsinks, ramsinks, windows, all kinds of mods you name it. I'm enjoying this SFF simplicity now, finally a system where I'm not removing side panels every other day - i'm actually working on the PC now.