Trainers always say not to use them, because they exert a slight pressure on your dog, and for a very well trained dog, just that amount of pressure should be enough to guide them - so in theory, at least, it trains them to pull against pressure, which you don't want.
Harnesses are not recommended either, because that gives your dog maximum leverage to pull against you - but of course, you have to consider the dog's delicate neck.
Having said all that, I generally use a retractable leash with a harness.
It's just a little bit of extra training as far as I'm concerned. Thor always walks by my side, or, if the leash is on its retractable setting, he can stop behind me to sniff.
I like retractables a lot because they are essentially a short and long lead in one - as others have said, you cut just lock the handle if you want a fixed lead. If I want Thor to Heel, I lock the Handle and say "Heel". He knows while it's on to stay focused on walking, and when it's flexi, he knows he can roam around a bit.
I think one last thing to consider is that it's possible for the retractor to break. I'm not all that concerned because Thor is 4 lbs and is not putting a tremendous amount of pressure on it, but I can see for a big, powerful dog, that could be a problem.
One final thought - if you're walking around other people, stay aware of whether you've created a trip wire between you and your dog.