I"ve built four or five PCs over the past 10 years, I'm a hobbyist not a pro. One was an HTPC build. I"ve always used mid-tower cases with ATX builds. I've always used negative pressure with no trouble, other than having to vacuum them out a couple of times a year. My latest case, Antec Sonata, has a dust filter and is super clean inside.
So typically, I have the PSU fan, the chip fan and one fan pulling out the back of the case, up high. I block all other air openings except for one or two at the bottom front of the case. I use 120 mm fans. Never had any trouble. The goal is to keep the PC as quiet as is possible.
This time I want to build an HTPC that looks like a set top box. I don't see why a negative pressure scenario won't work with it as well. And if I were to get an ATX case (I might go micro ATX, not sure yet, but my experience tells me that I would want the expansion) and put in some cards, will there be enough room to work in there?
What are the pros and cons of negative pressure in a media case?
How difficult are they to work in?
Are micro ATX components built, minimally to the same standard as regular ATX components?