9 speed works about as well as 8 speed which preceded it and 6 speed which predated that (4 and 5 cogs in back predate when I started riding road bikes).

With less bearing surface I expect lower life (back-of-the-envelope arithmetic suggests 20% moving from 9 to 10 cogs) but I can't believe that a side plate reduction from 2.25 to 1.8mm (I'm assuming based on 6.8/5.9mm external widths and 3/32" internal width) and couple mm increase in cassette width (Campagnolo 10 cog setups have the last hanging off the end of the freehub body) is going to cause things to stop working where similar moves from older gear did not.

My first 10 cog chain is fine after the first year and rainy season. If it ceases to be fine I'm out $36 which is not an interesting number divided by a year ($36/12 = $3/month) or longer.