I'm a fan of the kneelons -- I've owned two pair
(yes, you can wear out the neoprene, if you try hard enough!). Strange thing is, though, I don't use them much any more. Essentially, I now treat them like a super-duper athletic knee wrap, rather than as a daily-use tool.
I started using them to cushion my knees when I was an uchi-deshi at my old dojo. Working out 5-6 days a week, as a Sankyu and then Nikyu, the knee-pain started to get to me. And that got habit forming: I would wear them for every class. I found out, after a while, that they negatively altered my ukemi and my techniques -- I'm able to put more weight on my forward knees, and do so (rather than remain centered), and I started to rely on the padding to cushion my knees during a fall, rather than relying on good ukemi form. Basically, they encouraged me to get sloppy.
Since my knees were no longer really bothering me, I stopped wearing them. And both my ukemi and suwariwaza techniques seemed to improve.
Now I only use them when I'm sore and tired, and when pain in my knees seems to hold me back... As I get older, I foresee more frequent use.