This stuff I found is really good, it is silicone based (so has all the positives of the spray) but it also has a black tint which means longer periods of time between applications, or apply this once for badly faded bumpers then use standard silicone from then on to maintain the appearance.
Unfortunately I think many people are looking for a permanent cure for their faded bumpers, this I think is impossible (otherwise they wouldnt have faded in the first place). The above method is good for fixing the problem, but regular maintenance is required from then on.