If you have large hands, consider this grip as part of the purchase price of the G9. As a point of reference, my hand is about 9 inches from the wrist to the tip of the middle finger. As described, it does feel more like the traditional Logitech shape that many are used to. It reminds me of the MX1000 a bit. The default grips that came with the original mouse were okay for me. I was previously using the wide grip. When I use a mouse, I don't necessarily lay my palm on it. My palm usually lightly touches the mouse, but adds a lot of stability to my usage. The normal wide grip for me left me using my fingers only, which made me feel as if I had less control based on my style and fatigued my hand quicker. When I got this grip, it immediately felt home to me and I felt as if I had more control. The original grips with the G9 felt like a different experience that I was trying to get used to. Also, the side buttons seemed to line up better for me and "in the right place". With the wide grip, I needed to do some dexterity moves to hit the foward button.
The grip is a bit heavier than the default grips. I think it needs to be more solid since it is noticeably bigger than the default ones and this could be to make sure it does not flex at all. I noticed the difference in weight at first, but got used to it quick. Speaking of flex, none at all. This grip pops in and is very tight. I noticed some flex with the default wide grip (since I needed to use my fingers more than I am used to). Again, this grip is well made, of good quality and solid.
I did not customize, so I cannot opine on that. However, I took a conservative pattern and it looked really good. I will probably do a more gamer personal grip one day.
My only comment is the price. It is not a concern to me personally, but I can imagine that many people might pass over this grip because it is about $30 after including taxes and shipping. The biggest shame is that many of these people might have a completely better experience after getting it as I did. From owning the grip, as noted before, the quality is really good and it feels well made. I would suggest that maybe logitech lower the price a hair and offer free shipping to get the final price closer to $20 even.
In summary, this grip made the G9 go from pretty good to being glad that I have the mouse. I instantly noticed that I have better control and it felt closer to what I am used to.