Spoilered for many pics and so as to not railroad the discussion:
 
 
 In words, I love them!  I've got some rusty metal shelves in my garage that almost exactly match the tone of brown and orange so I'll vouch for the color, at least on some things.  I'm not sure if tanks rust exactly the same way as my shelves, but it's fairly convincing especially at a distance.  I usually apply 1-5 washes with 
TC (usually about 2 or 3) to get a little texture built up, then drybrush it heavily with 
RR.  And you can be messy and get off of the brown, or leave spots of brown without hitting them with orange too.  Really simple to do, and I'd say it looks a lot better than it deserves to for the ease of use.
 
 I'll also vouch for Blood for the Blood God, though I haven't had a chance to try the nurgle green one or the patina one, and I don't own the brown crackle stuff.