Overall, I am happy with the build, although a few disappointments along the way.
The first is the front and top filter material. Once I realised how much airflow these block, they had to come out. I prefer the appearance with the filters in - I don't like seeing the fans - but I can live with it. I was also surprised there was no manual in the case, although it wasn't too hard to figure out where everything went, and it was a pleasant case to build in, with plenty of room and well laid out.
I was annoyed not to be able to get my Noctua fan in the bottom slot, once I realised that the PSU - a Season X-1000 Platinum - is too long. At some point I may downsize the PSU, now that I am running only a single GPU.
Cable management is probably the best I have been able to do. The inside of the case looks pretty good, although underneath the rear panel there is a bird's nest of wire and the side panel is bulging a little. I can't imagine having even less space in the R3.
I installed the fans first, and connected them up to a spare molex connector, side by side with pc in my Antec P193v3, just to hear how the fans sounded. Very disappointing! The fans were clearly audible - even with the Noctuas using ULNA adapters, it was louder than the Antec with stock fans on low. The Fractal Silent Series fans also displayed a faint but noticeable whine.
Then I tried the NZXT Sentry Mix fan controller I had bought during the week, although my preference was not to install it. I wanted to keep the front of the case clean, and I also wanted to find some quiet fan settings using the Noctua adapters that I would never have to change. Unfortunately, that wasn't to be. The fan controller did solve the problem, however. With all fans on the lowest setting the pc was now virtually inaudible, and much quieter than the Antec. With all fans on full, the difference in noise levels is even greater than the Antec, and in fact the sound is quite tolerable.
As the pc is sitting on carpet, I am a little concerned that the PSU is taking in air from the bottom of the case, so the bottom filter will probably clog with dust pretty quickly. I may have to put the PC on a piece of wood to solve this problem.
Temps don't seem to have changed much. The CPU always remains cool thanks mainly to the Noctua NH-C14. I am testing GPU temps with Kombustor as I type this, and the GTX 680 seems to have stabilised at 79 degrees with fans on full, compared with 82 degrees in the Antec case. (Switching the top fans to lowest settings, leaving only the side fan and front fans on full only adds a single degree to the GPU temps but significantly reduces noise.) This is a marginal improvement over the Antec, and not quite the dramatic drop in temps I was hoping for. Until a few days ago I was running GTX 580s in SLI and the noise and heat - and the random crashes from overheating - were driving me crazy. Going to a single 680 has solved most of these problems (with a slight reduction in fps in BF3), but it doesn't look like this case will allow me to add a second 680 without encountering the same problems.
Aside from the cooling and noise benefits, I prefer the Arc Midi to the Antec case because it is smaller and better looking, it has no door (which I prefer) and the power switch and USB ports are on the top where I can reach them. Overall, I'm glad I switched to this case.