I had never used flux before in any of my soldering.... until today. All I have to say is wow!
I was trying to manually tin a board (fun stuff) and at first I just did it without any flux. I got through about one trace and though, wow, I have a ton of solder on there!
So I run upstairs to grab my just-shipped flux pen and tried it out.
I equate the rest of the tinning to "color by numbers". It was that easy. I just pushed the solder where I wanted and that's where it went. No bridging, no troubles at all.
Actually, do you remember those things you did when you were a kid, that all you had to do was get a certain part of the picture wet and it changed to the right color? Yeah, that's how easy it was. Sure, liquid tin would have been easier, but this was pretty easy (and it gives you a great opportunity to inspect your board closely.)