I recently converted my 8 speed Altus drive train to a 9 speed Deore drive train. The shifts at the back are much cleaner and you definately get good bang for your buck there. I also upgraded the front derailler from the bog standard 'sis' spec to a Deore to match the shifters. Again, an improvement - but not all that much. At the price its worth it for the easier shifting. The brakes were upgraded from no name cable operated discs to hydrolic deore units. Very happy with them at the price.
However, looking at xt/xtr level parts is there really an increase actual on the road comfort/performance that matches the double/quadruple tag or is the price just for the 'bling' factor and a bit less weight?