My first time through w/ room treatments was my parents basement, where I simply coated the front wall with auralex absorption panels. I believe they were 2' x 2' x 2", but they may have been 4" deep, I cant remember. It was not very sophisticated, but it did soak up that main reflection pretty well. They were hideous though, solid purple. I wish Auralex would make a decent shade of white at normal prices.
My second experience is my new house, I went with Acoustimac absorption panels, followed by Auralex diffusion panels. I think the Acoustimac stuff is similar to your GIK material, its fiberglass with a wood backing and coated with a cloth that allows penetration of sound. I went with 2 bass traps (2' x 4' x 4"), which are on wood risers, and several panels of the same size, that are only 2" deep. I forget how many, I want to say 4...
The Acousticmac cleaned up echo like you mentioned, but the most amazing improvement was when I added the diffusion to my back wall. Its a very small room, and the back wall is only several feet behind the computer desk, so its not surprising I guess. These are 2' x 2' x2" "MetroFusors". It cleaned up echo, and seemed to give everything much more definition across the entire range. After that, I started (but have not finished) installing Auralex mini fusors on the top of the 2 sidewalls. I also placed polyfill in these to catch the sound that penetrates the resin (mainly bass frequencies afaik). My goal was diffusion, but I figured the polyfill could help even out the bass since it tends to collect in corners.
If I stopped now, I'd be happy with the sound, but the diffusion made such an improvement that I want to experiment further. I'm probably going to get several more absorption panels and try mounting them on the ceiling, and some sort of diffusion for the remainder of the ceiling. One thing I'm curious about is mounting techniques for the absorption panels. I'd like to try mounting at an angle to create an air gap behind them, but its tough to do while maintaining a desirable aesthetic.
Room treatments definitely gave me a deeper appreciation for speakers overall, as the room becomes 'energized' in such an exciting way. I was surprised at the sound quality I've gotten with my new subwoofer, since it was the cheapest model in REL's product line. I'm guessing the treatments share in the results quite a bit, even though I've not really properly utilized the largest panels.