Well blowing up HD was mainly experimental stuff I tried. I'm not a pixel peeper but sometimes I really like a shot and want to use it as a frame grab still! So when I go to print I usually like to be at >3K...some of my HD blow ups turn out great and some don't, depends on the shot really but one of my DVD covers is a frame grab, its not a very large image but its very sharp for its size. Another example is creating a 4K Digital cinema package when you say only shot at 2.8K with some 4K film scans and RED footage thrown in (something like Argo) But yeah for the most part I agree it is pointless and resolution isn't even have the battle in the end of things. I've seen 720P only films that look WAYYY better than 1080P stuff, heck I've seen SD stuff look better than 4K stuff!
As for full frame vs crop detail, I've taken some great shots with my Canon 550D and others have taken even better shots with their APS-C cameras no doubt so sensor size =/= better image, comparing a 7D and a 5DII I think would be fruitless in terms of extra detail...I just feel full frame is what 35mm film was and thus it should be the standard. The field of view is just unique and upgrading to full frame isn't soley about a bigger FoV, if it is then your priority's might be out of order, with the amount spent on the 6D I could have gotten a nice 70-200 or better yet two new Canon L lenses I've been looking at to help complete my kit: The 135mm /2L or a wide zoom (probably a wide zoom), the 200mm /2.8L for good reach. That would be gotten rid of pretty much all my issues in terms of high speed lenses that can reach out and touch with ease as thus far I've been using much shorter than normal focal length glass, stopping down for sharpness and making my photos from crops! BUT I think a full frame camera is a better investment for now, I tend to only need 1 or two focal lengths at any given time/event...I usually walk around with my camera with my 35mm on and my 85mm in my bag...I have a 35mm 2, 50mm 1.8 and an 85mm 1.8 very happy with all of these lenses and while I "lose" some reach using the 85 on my 6D I can crop like I'm use to and most likely get similar quality....otherwise I can use my 550D with the 85mm mounted as my long shot camera and my 6D with my 50mm or 35mm as my standard...Ill have to experiment with that combo once the damn camera gets to my house
Only then will I be cool having two expensive DSLR kits on my person at once. Anyways I can rent a 70-200 2.8L for special occasions of course.If I could have a 6D with an APS-H sensor with the same low light performance of the 6D I'd be happy, I think APS-H (1.28x crop on Canon) is the perfect intermediate. I would grab it in a heart beat! I'm a huge APS-H fan for many reasons, I was a sucker for some of the footage off the 1D Mk.IV XD. Sadly I don't see it making a come back with Canon but I hope to be proven wrong! That said Pentax released an APS-C camera with an insane sensor that beats some full frames. So regardless of sensor size (so long as we are around Super35 or bigger!) I hope low light performance, color and detail keeping getting better! Gotta love/hate technology!
As for 6x6...first time I saw a 6x6 camera I was confused, hate the square aspect ratio though...me and a friend were exploring an old cement factory and naturally that meant we brought like 5 different cameras (he collects cameras), so we got on the roof of the place real high up and you can see this vast array of buildings, highways, trees, railroad tracks and I remember thinking "This is something worthy of anamorphic 70mm". Maybe growing up I just associated vast landscapes with tons of stuff in view with wide screen but I feel the square aspect ratio doesn't do certain scenes justice. I think the square aspect ratio is fine for bill boards, fashion and really anything you want to blow up GIANT, but for an immersive scene wide screen is just the way to go, but thats my view. I posted about this yesterday on Cinematography.com...me and a few other people wanted to grab the AG7200 anamorphic adapter (only 1.33x squeeze factor but still cool) and shoot some anamorphic medium format with ultra shallow depth of view enhanced with the anamorphic factor in. Of course you can crop tell you drop with medium format but anamorphic has a different look and thus a different emotion to it. Given the choice of shooting an entire movie in anamorphic or spherical 8/10 times I'm gonna have to say anamorphic unless it wouldn't work too well with say the era...for example I'm doing a short film that takes place in the 30s and 40s so I plan to frame for 1.66:1...would have done 1.37:1 but I hate square video with a passion.