Background:
I needed a slider to accommodate my Canon 5d setup with an average-weighted lens.  The standard Motion Slider series was one of the few that could support such weights. 

Setup/First impression:
Right out of the box, I mounted the slider to a Manfrotto 057 leg without a head.  I was expecting the slider to teter, but saw no such movement at all.  I first tested the mechanics with a small compact Canon S95, and the slider worked great without any practice. 

Test with a 5DII and Lens:
A test with the weighty 5DII and 2.8 zoom lens required more effort and practice.  During use, the slider showed some slight dip in height on the ends as a result of the extra weight.  This weakness is more likely the fault of the tripod setup (and not the slider itself flexing).  A dual tripod setup would have resolved this, but the Motion Slider 36 can only accomodate 1 tripod mount.  The longer Motion Sliders can accomodate 2 tripod supports.  The 36" model seems to be right at the limit for a single support setup, if not a little over.  The "dip" should not be as perceivable on wide angle lenses, but likely more noticeable with telephotos that have a noticeable horizon reference.   In the latter situation, you may want to consider putting the slider on a table or sawhorse (strapped at ends) instead of a tripod.  To add more control of angles, a video pan head is recommended between the slider and the camera...the Motion Slider folks recommend a Manfrotto 701HDV head.  I would not recommend inserting a head (i.e. ball or pan) between the slider and the tripod support as I think it would introduce more instability and tetering...but I have heard other users successfully doing that.

Results:
The slide motion itself is very smooth with its dual-track design.  I have not compared to other sliders, but the sliding sound is detectable, but not loud.  Most of my applications would not be recording sound, so this was not a big deal for me either way.

Weaknesses:
Minor: The thumb screw that secures the camera to the slider (not the base) is difficult to use, and likely even more difficult for larger hands.  The workaround is to secure the camera as best you can, then do the torquing from the camera, not the screw to tighten.
Moderate: With an aluminum construction, the main mounting hole to the tripod base can potentially strip with repeated use or if the user does not mount the tripod carefully.  It would be nice to replace or append a durable thread solution.  The bottom of the slider base should be mounted directly to as much surface as possible in order to get a sturdy setup.  I have one workaround suggestion for the user below.

Suggestions for the user:

With repeated mounting and dismounting of the slider to a tripod, the mounting hole on the base can potentially strip.  My recommendation is to install an extra tripod disc (e.g. part R055,739 for Manfrotto 57 legs) to the hole and leave it there.  This will transfer any potential wear to your tripod leg column setup, which presumably should be much more durable.  The downside is that you may have a protruding bolt on the slider which could potentially puncture your bag.  A bag is recommended to keep dirt and dust out of the tracks.

The slider moves easily, so if you did not order a brake with your slider, you will need a way to secure the slider when not in use.  I use a velcro strap looped through a rubber band to keep the slider at one end of the base.  Be especially careful of a camera that is not braked when mounted...if you are adjusting your tripod legs, the camera can travel freely and pick up speed, alter the center-of gravity, and potentially knock your setup to the floor.

Recommended: Yes