largely along the thinking that highlandsflyer pointed out that in order to get that many gears within the same space the cogs have to be much thinner so I wonder if they are built to last or just fail regulary to keep you spending?
Well, I read on a local forum that the 2011 XT 10-speed kit on my modern has a life expectancy of 5000 km (around 3000 miles) for chain and cogs ... assuming you have 3 chains and rotate between them every few hundred miles.
With 1 chain, expect the chain and cassette to be completely worn in less than 2000 miles.
To give you an idea :
On the left is the chain of my Copperhead. The bike has done about 500 miles since I bought it new in August 2011.
I'll occasionally accelerate hard and ride fast, but overall I'm being pretty careful still because the bike isn't really finished yet.
On the right is the chain of my Sbike 503. This is the original HG50 chain that the bike left the factory with in 1993.
Worn wheel bearings and the sorry condition of the rest of the bike make me think it had done somewhere in the 20K+ miles region by the time I bought it. The drivetrain had been completely neglected for a decade too.
I have put around 2.5K miles on the Sbike since I bought it in October 2011, and the bike is ridden a lot
more aggressively than my Bulls.
As you can see, both still pass the test. However the tool's exact contact point with the rollers indicates that the modern chain already has slightly more wear than the old one. The measuring tool really goes further down in the modern chain, which indicates that it's stretching already.
If that HG50 doesn't break, it'll probably last another 10-15K miles. However that 10-speed affair will probably need to be replaced this time next year.
Still, lower-end Shimano stuff has always lasted longer, and it's no different for modern parts. a 10-speed SLX groupset seems to last 3 or 4 times as long as its XT counterpart.