This is my review of the Opteka GDL-400 Camera Slider.
For its price, it is a pretty decent slider. The slider itself moves smoothly along the rail. There is, however, a slight wobble caused, I guess, by some 'play' between the slider and the rail. This happens when a heavy lens is used and the camera's center of gravity is not in the middle. It could be avoided by mounting the camera on the slider in a different way, a way which allows to change the position of the camera. When using a long lens, this wobble is magnified, so it is an issue to take into account. The wobble can be avoided by pressing on the slider carriage while sliding.
The 120cm long rail is quite firm and when used with its included metal feet, it is very stable. Mounted on the the Manfrotto tripod and video head, the construction is stable, even when the camera is slid completely to the end of the rail. There is, however some 'play' then and the only way to avoid this is to add a second tripod or monopod. This play is not caused by the rail that bends, but it is the tripod/videohead that causes it.
There is a screw for fixing the slider carriage for transport, but I do really miss a bag.

Gear used:
opteka GDL-400 Camera Slider (120cm / 47'')
Tripod:Manfrotto 055XPROB
Videohead: Manfrotto 501HDV
Manfrotto PL501 quick release plate (to mount the rail to the video head)
Cullmann Cross CB5.1 ballhead (to mount the camera to the slider carriage)
Canon 550D/T2i
Lens: Samyang 14mm F2.8