About to invest in room treatments; GIK, RealTraps, DIY -- what is your experience?

I have ASC, GIK, Michael Greene, and acoustic field products in my room. I have anywhere from sub-bass absorbers to diffusers. Both GIK and acoustic field offer diy products and videos to build your pieces. I bought the 8 dvd set from acoustic fields to build pieces. Any of the above mentioned companies will work with you on what you need to treat your room. Also, I think argent is no longer in business. 

Check out Acoustic Sciences Corp ..  https://www.acousticsciences.com/   Their stuff has been an integral part of my system since 1993.  https://www.martindareff.com/Other/HDR-Treated-Images/i-ZmLg23s/A

With regard to diffusion vs absorption with GIK panels, it costs only a little more to hedge your bets. I was initially skeptical about diffusion, since I was trying to tame a very bright room. I ordered Alpha panels with the optional cloth finished back so that they can be used for diffusion + some absorption or pure absorption, depending on which side faces into the room. I found that, in my room, the diffusion was noticeably better.

I have a total of nine GIK panels (3 Alphas, 3 non-membrane bass traps, 3 membrane bass traps) and haven’t encountered any QA issues, other than the little adhesive backed GIK labels eventually falling off the cloth covered panels.

I initially sent room measurements taken with a calibrated USB microphone and REW to GIK for recommendations to start off the process.

As expected, the improved reverberation time makes the room act as if it is much larger. Speakers (Sony ES SS-M7) generally considered to need lots of space to work well never sounded better...

I have a similar ceiling height.  I got Vicoustic bass traps and ceiling panels at first.  I was ready to get some wall panels, but Vicoustic kept changing their design for hanging/adhering to the wall.  After 4 months of waiting my dealer gave me a refund and so I bought directly from GIK.  I agree with martin0820 - the diffusion is just a little upcharge over absorption.  If I could do it all over again I would have gone with all GIK.

I had a great experience with GIK. I chose them as well-established science based and reasonably priced but was tempted by synergistic research stuff. GIK = good tools, good advice, and great results. I have a vaulted ceiling with speakers along long wall and top of vault along center of room in long direction. Ended up with 2 244s on ceiling, soffit traps stacked in front corners, etc. 14 piece total but only used 12. Dealer said he couldn't hear my room at all which was great. Not dead sounding at all.