My first all grain was a triple decocted Hefeweizen (but using non-traditional rest temperatures). The only reason for 3 decoctions was that I have no other means of increasing rest temperatures. While I would not recommend this for beginners, I learnt a hell of a lot from the process, and was able to gain a really good "feel" for the stages a mash goes through (look, texture, smell, taste) as temperature rises. I deliberately avoided resting too long and protein temps, as I was concerned about excessive protein degradation given the well modified malts of today.
As an experiment for my last batch, I decided to go with the traditional 50/60/70C (or thereabouts) schedule, but compensated by increasing the proportion of wheat malt from 50% to 60%. I managed to get the number of decoctions down to 2, using boiling water for the remaining temperature adjustments. Hydrometer samples from the fermenter are tasting pretty good so far; will be interesting to see whether head retention is affected.
I can't say whether decoction makes a big difference for lager styles as I've not yet brewed one, but I believe that Hefeweizen is a style that is profoundly affected by the process. My own opinion is that it is the boiling of the grains that has the biggest effect on flavour, for reasons already stated by others in the thread.
With respect to Kai's original point about no or very low boil; this is the approach I have always taken (mainly through ignorance). I typically only stir on high flame when getting the decoction up to rest temperature or boiling (with the lid on), then throttle right back for the boil (effectively simmering). Only need to stir during ramp up, and occasionally once it is boiling. Have not had any problems with scalding or sticking as yet.