I've got a 2006 Kona Caldera, after I upgraded from Giant Rincon Disc. It's 2011 now and I'm still riding it.
It's a very good bike, well worth the £600 or £700 that I paid for it.
I did a Trans Carpathian Marathon on it and it performed very well!

Over the years I didn't have any unexpected problems - things that I replaced and reasons for it were:

1) pedals - I wanted clip in ones
2) head bearings gave up after three years
3) bottom bracket gave up after three years
4) upgraded brakes to Shimano XT as I had some spare cash
5) hubs started being loose after two years, so upgraded wheels as at that point I didn't know how to tighten/adjust wheels.

I have still got original frame, gears, seatpost, stem, handlebars. Fork has just started to make noise so I might be replacing it this year.

I have also used this bike for commuting after swapping tyres to slicks and although the handlebar is quite wide (66cm) it performed really well.

I must say though that as I was looking at Calderas from 2007/2008/2009/2010 I think 2006 had the best groupset quality/price ratio.