Hi Mike,
I think the folks here have given you some very good advice. I'll chime in as well.
The GIK products are effective and very cost effective. I have 4 of their corner bass traps called Tri Traps. I would tell you that they work as advertised, but as others have mentioned, they lack the absolute best fit and finish compared to products from ASC or RPG.
I also have products from RPG - I use a variety of their BAD panels which I find very effective as well with generally high quality fit and finish. I also use their Skyline diffusors on my ceiling, painted by RPG with the same paint used on my ceiling so they blend in better. The downside of RPG is that they are on the expensive side of acoustic treatments and they take a while to deliver your order once it is placed compared to the some of the other acoustic treatment companies.
I also use bass traps from Real Traps - specifically their corner Mondo traps. Again, a very effective product with good fit and finish. Another company that hasn't been mentioned yet is called Kinetics Noise Control. They make a variety of acoustic products for all types of solutions including for home audio/theater usage. They make a specific product called a TAD panel. It is basically identical to the RPG BAD panel both in terms of looks, function and fit and finish and about the same price. I use some of these as well. The advantage of Kinetics compared to RPG is that they deliver your order once it is placed much, much faster than RPG: 2-3 weeks versus 2-3 months from RPG. I have ordered twice from each of these companies and the delivery times were consistent.
I have been in several rooms of friends who use ASC products. Again, they are effective once you have dialed them in properly. But, you need to realize they generally sit on your floor and extend out from the wall quite a bit depending on which size you get so they tend to eat up your floor space versus wall or corner mounted panels.
Bottom line, there are a good number of companies to choose from - which one to use for me was a decision based on which one had the product that fit the problem that I was trying to solve - both from an acoustics point of view as well as the look of the treatment itself. Take the advice as already given - start slow - bass traps in your corners and treat your first points of reflection and learn from that. You may need to add additional treatments beyond that depending on your room and system, but add them sparingly as you do not what to over dampen your room.