I installed this spring guide in my Echo 1 G36C, and it was probably one of the best upgrades I did.
It is heavy and extremely solid, which gives a comfortable feeling when installing it.
It is full metal, and bearings spin freely. The bearings help prevent the spring from twisting and coiling up, which can result in FPS loss. The bearings also act as a spring spacer, resulting in a boost of ~10 FPS per shot.
The only complaint that I have is that the hole on the back of the spring guide is too small for a decent sized screw driver to fit. We all know that there's the possibility of receiving a spring guide in the face whenever we open the gearbox. This is avoided by sticking a scredriver in the hole on the back of the spring guide and pushing on the screwdriver to keep the spring guide from flying out. However, on the Echo 1 version, the hole only allows a small screwdriver to be put in. This means that it is harder to control the spring guide and put force on it. It is a pain, but shouldn't pose a problem if you don't open your gearbox that much.
There isn't much to say about a spring guide, for they are all basically the same in form and function. This spring guide does its job, and is worth the 13 bucks.
Bearings spin freely
Hefty and solid (full metal)
Bearings act as a spacer, resulting in a ~10 FPS boost
Hole on the back of the spring guide is too small to fit a large screwdriver in