I love the weathered look as long as not overdone - which I tend to do just a bit when I do it myself. I think AMCDave's decapod is maybe the best weathering I have seen. Very subtle but clearly there: this is a locomotive that is in service and working but it well-cared for and in good condition, just not showroom clean. I think most locos were like that.
I need to get a good airbrush and learn to apply the dustlike dulling that Dave did that that, etc.
The Legacy Berkshire below is what I regard as my best weathering attempt. I had put a more prototypical cab and a few other tiny changes on it and had to repaint it after so I decided to weather it too. I began with Neolube on the drivers and valve/running gear - "blackening" it without making it too black. That helps a lot -- note AMCDave's decapod also got this treatment. Not obvious (that is the point of weathering, I think) but I used Neolube a bit on the cylinders, firebox (just a trace, and elsewhere. Then I used brush powders (not nearly as good as an airbrush I think) to dull and lightly weather up front , along the tender bottom sides and trucks, etc. I understand the realism of the "white deposit tendrils" down the boiler sides that come about when locos with hard water, particularly in the western US, but I don't like the look. I did just a trace here: I've stringy trails of the stuff looking thick and white as bat guano in some old photos - Ugh. That is too real for me