I have bought some Mystic Stitch patterns, but as yet have not tried one. I do have a close family member who is working on one (200 x 250), and I'm able to see the progress almost daily. It's absolutely beautiful.
Here are my personal comparisons:
Artecy--I finished a large one (300 x 218) a few months ago. I loved the experience and the finished product. People were also amazed at how detailed it was. My only regret? Since it was my first large one and I had only worked on 14 ct fabric up to that point, I did it on 14 ct. I've since started projects on 18 ct, which, to me, create a more seamless look. But of course, that's totally on me, not on Artecy.
Scarlet Quince--One of the 4 projects I'm currently rotating is from SQ. It's 405 x 288. Again, it's another breathtaking pattern. SQ is known for using blended threads, which creates an insane level of detail, but also an insane amount of color changes!! :) Regardless, seeing the piece come alive is such an amazing experience. It's going to take me forever to finish, but who cares?!?! :) The only "complaint" I have (which is not really a complaint) is that the regular-sized charts are small. My eyes (plus glasses) are fine but it's a lot of info to take in at one time. They do offer large-sized charts, which I'll probably buy the next time I get an SQ pattern.
Etsy--Naturally, Etsy is not a dedicated cross-stitch shop, but there are PLENTY of large patterns available there that are gorgeous. The one that I'm working on is 304 x 350. And it's actually my favorite one to do right now. The designer put the colors together beautifully and it's coming together so nicely.
Now for my simple opinion rather than experience (haha!) :)
HAED--Haven't worked on one, but I got a large 87 page freebie off their site recently and the pattern alone looks stunning. I can only imagine what the finished piece looks like.
StitchX--I started a medium-sized one, but didn't finish (for some reason I got bored with the project, but I intend to return to it one day). I love their patterns, but I don't always like the symbols--depends on the pattern. I have 7 new ones that I need to work on one day. Too much on my plate!!
Shinysun's--I've only seen a sample page of their patterns, but I'm going to bet, based on the patterns they offer on their site, and what I saw on their sample pattern page, that their large patterns are just as wonderful as some of the more major designers.
Ok! Phew! Long post! Sorry! But I do hope it helps!! :)