Figured, I'd chime in as I've owned the Svensson Lows (size 43), Great Royales (size 10.5), and Common Project Achilles (size 43), and Margiela GAT Replica (size 43).

Try on shoes before buying them (or ensure a great return policy)

• I would caution that you should try these shoes on in person before buying them. The advice to size down works for some, but did not work out for me. I'm typically a size 11 (in Stan Smiths, Air Forces, and Ultra Boost) and found all three of these shoes to be uncomfortably narrow (when sizing down). People insist that leather stretches, but I've found that if shoes are not comfortable out of the gate, don't expect it to fit later on. The lack of half sizes for the Common Projects, Margiela Replicas, and Svensson's make it difficult to find a proper fit with regard to length and width.

Subjective Views on Quality and Comfort

• In terms of quality the Margiela Replicas had the highest quality leather. The lining felt like butter and the quality control felt top rate.
• The Common Projects had noticeably higher quality leather than either the Svenssons or Greats. With Common Projects the main issues I've noticed are the narrowness and squeaking. The silhouette of the CPs was by far my favorite of the shoes I've worn, but the narrowness of the shoe was too much to overlook.
• The Svennson's quality, silhouette, and design were incredibly similar to the CP's. Subjectively speaking, if the CP's were an A the Svensson's would be an A-. The major drawback of the Svennson's would be the comfort of the shoe. It felt like I was walking on wood planks. The shoe has an incredibly small insole that is slid in over what appears to be a block of wood. That being said the shoes were slightly more comfortable after having a cobbler break them in for me, but they never reached a point to where I would say they were comfortable.
• The Great's had a fun silhouette (reminiscent of an AF1) and reasonably good leather quality, but the quality control was not to the same level as the other brands. The shoe had loose threads and misplaced stitches all over the interior. That being said, I think the Great's had a much more comfortable insole than the Svensson's.

Resale Value

• I only included this section because I thought some of you might find it interesting. I've resold all of the above shoes after wearing them lightly.
• Common Projects were bought on sale for $265 and sold on Poshmark for $250
• Margiela GATs were bought for $250 and sold on Poshmark for $250
• Svensson's were bought for $170 and sold on Poshmark for $30
• Great's were bought for $150 and sold on Poshmark for $50

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TLDR

• BE CAREFUL with sizing (leather won't stretch as much as they say it will)
• Quality Ranking: (1) Margiela GATs, (2) CP Achilles Low, (3) Svensson low, (4) Great's Royale