I'm no fan of NLP - it was intended as a personal development aid to deal with phobias and disorders, so was intended to be internally-focused. However, in my experience, many use it as a manipulative tool (externally-focused), which I dislike intensely. Makes me feel I'm being played, so recognising that is the value I derive from understanding it.
Dale Carnegie's book, however, is (my opinion) the best and most useful personal development book I've ever read.
That's not a problem of the tool itself, but a problem with the handlers of the tool.
Usually people learn NPL to fix weird behavior within themselves. Once that happens, they usually ignore the meme "with great power comes great responsibility", and go their way messing with other people's ideas, or even feelings.
If you learn it as it was intended, and refrain yourself of exploiting it for profit, you're on the good side.
I always wanted to learn NPL, but I've never tried because I figured I wasn't ready yet. But after this, I think the time might be perfect to pick it up and weed out some weird behaviors of my own.