I use a temperature controlled iron.  I typically leave it right at 600*F.

I note that the 15W iron doesn't even idle that high.  I notice that if 
I accidentally end up with the iron set much below 600 - like around 
500*F, I definitely notice the difference.  

The 25W iron seems to be right around 600*F - starts out hotter, but 
works down.   Which is interesting since I used to prefer a 25 or 30W 
back before I got converted to the temperature controlled iron.

The 40W iron is way too hot.

When I started soldering, I used the radio shack irons exclusively.  
Mind you, this was like 25+ years ago, and temperature controlled irons 
weren't all that readily available.

There were two major things that I learned which made a big 
difference.   The first, is the benefit of a well, cleaned tip.   The 
video mentions this at the end, and it can't be overstated.   A sponge 
or one of the "shredded brass" cleaners is essential.   A clean iron 
with a tip which is tinned all the way around and all the way from the 
tip of the tip to an area a bit back from the works 100% better than an 
old, abused iron.   Regardless of the wattage.

The second is the benefits of a temperature controlled iron.     With a 
temperature controlled iron, you don't generally have to worry about 
things like how many joints you have soldered since you picked it up.   
Of course, you occasionally will outstrip the iron's ability to produce 
heat, but I will say that with a good temperature controlled iron (which 
aren't that expensive), soldering becomes a very repeatable and reliable 
process.

-forrest
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