Well, first of all I should say this was quite a project (only my second build-it-yourself PC and I also had to order 4 different components from Amazon - PSU, DVD writer, PC case and a slimline adapter); I bought all the essential parts first: CPU, PSU, RAM, SSD, etc.. then 2 weeks later I got the GPU and then one month later I got the DVD writer.
A small anecdote: Originally, I had bought an r9 nano (sapphire) but the one I got (like many others apparently) had absolutely unbearable coil whine, and the fan was also extremely loud to boot. Luckily, that was the last one the retailer that I bought it from had and so I was given a full refund. Eventually I settled for a weaker GPU as you can see in my parts list (will expand on that in a jiffy).
The idea for this build was to have a tiny and silent PC (for general-use and some gaming) on my desk right next to my monitor which would make certain things more convenient such as cleaning the PC itself, cleaning the floor underneath my desk and also saving some space in my room.
Now let me share my experience with building and, more importantly, using this PC:
CPU: Nothing too interesting here. works flawlessly although I couldn't overclock it much (4.2 stable) but I guess that was to be expected considering there are no case fans and only a low-profile CPU cooler.
CPU Cooler: This one is literally mute at low to medium load. Super-easy installation as well. very satisfied.
MOBO: Well, here I can't really say much other than "It works" since I don't care about most of it's features. Do note that some motherboards could have compatibility issues because of the USB cable and the PSU being right in-front of it's socket on the mobo (see gigabyte h170n, z170 and z170 gaming).
RAM: Cheapest ddr4 I could find. does it's job and does it well (looks nice too though you probably won't see it with all the cable clutter :) ). fits perfectly with the l9i cooler.
SSD: Moved from an old HDD. ahh the good life :D. Note: This PSU has a sata cable which has reversed connections but I was still able to attach the SSD to the front of the case. It looks a bit crooked but it feels OK I think. I'm afraid to try and unplug it, however :0.
GPU: For a mid-tier GPU this one is quite powerful. It runs The Witcher 3 at 1440p, 30-40 FPS with a customized med-high settings. An important note: The lian li pc-q21 has a huge problem in terms of GPU compatibility due to it's width. the only 970 mini model that might fit in this case, without the PCI-E cables preventing the case's side panel from closing, is the one from inno3d (asus and gigabyte 970 mini are too wide for this case). Thankfully AMD has small GPUs with the PCI-E at the front, like the one I ended up getting. I'm pretty worried about future upgrades because of this :(.
CASE: One of the smallest in the business that can house a "fat" GPU. It looks VERY small when you see it in person. Cooling isn't very good however - the CPU reaches 70c-80c while gaming and the GPU can't be overclocked without having it's fan go beyond 55% speed(around 2500 rpm) which is too noisy (for me at least). Honestly, building inside this case isn't as hard as it might look. Note: Clean it with 70% alcohol to remove fingerprints!
PSU: A very high quality PSU. fairly silent at load. very short cables (great in this case). Note: The cables are very stiff and VERY hard to plug\unplug. Also see the note under "SSD".
DVD: Just got it today. The adapter I got allows me to spare the damnable molex cable and use one of the sata cables included with the mobo :D.
To summarize: I'm very satisfied with the "general-use" aspect but if you're looking to build a gaming PC with maximum performance, you should probably avoid building something like this (heat and noise will be thy worst enemies).