The KMC X9SL Superlite 9 Speed Chain has the same features as the X9 chain but adds hollow pins and cut outs on inner and outer plates creating a super lightweight chain.
I will preface this review by stating that I am fairly low weight rider (145 Lbs or so) and I do keep my drivetrain pretty clean. So while I won't put a chain under as much stress as others I do tend to ride fairly hard. This would be my third KMC SL chain, all them on mountain bikes. First was a Ti coated 9 speed on my HT S-Works Stumpjumper which saw mountain biking duty as well as commuting, that chain probably saw 3,000 miles. Second and third where a 10 speed SL on Niner Air 9 and the replacement for my Stumpjumper both went on at the beginning of the summer and have been doing great.