Heya, I've commissioned a lot of art recently and I think it's always good to know how an artist treats their clients.
These are completely my own experience and should not reflect the artist as whole. People change, have good days, and have bad days.
Do not, by any means, harass any artist based on what I say here. I am not an expert, my word should not be a deal breaker but more or less considered.

Recommend:
 O.Z.Y.
Beautiful art at a reasonable price. Kept me informed about the process and constantly sent updates and explained the process to me all while being polite and friendly.
 BeastFromHell
Super cool person to talk too. Obviously had fun with the piece and even threw in a colored version for free! Art is beautifully well done, with excellent focus on mood, colors, and shading.
 1CASSIUS1
Beautiful art at an amazing price. Does not send progress images on offline sketches but more than happy to fix mistakes or make tweaks at the end. Talented person, equally talented art.
 SK1TCHY
We exchanged pictures of pets, need I say more? Great art with sharp lines and super fun to talk too!
 sorrowkeeper
High quality, interesting, and traditionally beautiful looking art at an affordable price and a quick turn around time. They where very kind and updated with progress often. Perfect person if you want some renaissance vibes. Even drew me something extra once my commission was done!
 suulfur
Amazing grungy pieces with great prices. Kept me updated every step of the way and was understanding and polite to talk too. You can tell that they enjoy what they draw.
 Sergevna
Great and interesting YCH's with cool angles at a fair price. Also does amazing work with lighting and shading!

Do Not Recommend:
 ahazai
Seemingly took on tons of slots for a YCH all at once then disappeared for about 8 months and dodged DM's about the progress. Came back, sent sketches and I asked for a slight tweak. My request was ignored and the piece was finished almost 1.5 years after I had commissioned it.
 Tres-Art /  Nightcrauzer
Likely due to misunderstandings and difference in language, but Nightcrauzer was very blunt, rude, and tough to work with. Tres uses them as a middle man so they are not flooded with DM's but the process was made difficult and hard to follow with no input from Tres at all. I understand the use of a manager, but I don't think it should've been this difficult to communicate what I wanted.
 BlakixTekzer
Seemingly keeps commissions open permanently, but queue is over a year and a half long without any updates. Would have liked to have been warned of that before paying for a YCH. However, they do have a public list with WIP's of their queue.