I think that you have to not only define a "negative" or the eccentric motion but also under what conditions you apply it.
Are you doing negatives after normal work sets, doing negative reps at the end of the work set, or doing a separate eccentric workout? Are you truly doing little to none of the eccentric work? Are you doing it during cycle or out of cycle?
As Massive G pointed out, it is difficult to get people to truly spot for a totally eccentric set where they totally lift the weight back into position. Because of this, I primarily only did them if I could coerce training partners into participating as well.
During the pre- interweb days when info was more scarce and anecdotal in nature, "negatives" were my "secret sauce" to overcoming stagnation.I rarely saw anyone teeing up a totally eccentric set.
There is no doubt it was the key to jumpstarting gains for me that showed up for sure in the strength department. I have used it more than once on younger guys trying to overcome strength hurdles in the workout.
It is pretty non intuitive, because I would not feel like I had a heavy workout in the traditional sense, but it certainly required a bit of recovery.
While a little bit of my experiences with it could be chalked up to "Anything different works for the short term" I truly feel like it was a valuable tool to have, mentally, if nothing else.
PWood