I have bumpers in my GSS necklace and it hangs very comfortably, but not
quite as loosely as knots.
Edit: See my post #158 below. It turns out that the wire on which my GSS are strung is the cause of the lower flexibility. When
thread is used as the stringing material, the bumpers do
not reduce flexibility; I've tested it.
The bumpers, being lenticular in shape, are a tiny bit narrower than a knot. So if you have a necklace that hangs just a
wee bit too long, bumpers may help it to shorten it that little bit. I am thinking of restringing my WSS necklace with bumpers for that reason.
I have found that with heaver pearls in particular, small gaps eventually show up even when stringing with Serafil or Beader's Secret. It's not that the thread stretches-- I believe the knots just get tighter over time, thanks to the weight of the pearls. I think the bumpers may help with this problem as well.
Just one caveat: knots also serve to secure the pearls in case the thread breaks. Bumpers will not serve this purpose. However, I have never had a thread break. And Serafil or Beader's Secret is quite strong-- 24# when doubled. Usually I end up restringing periodically anyway, when the gimp starts to look tatty.