Some people find it an exercise in showing off, or simply in padding. And a lot of the tips to make the word count are very bad. They will not produce a good book. Some are offended because it encourages anyone and everyone to write, even those who are not committed heart and soul to writing.
I find it a splendid excuse and pound out my words. A little competition, a race against a clock and deadline, makes me more productive.
I was an early adopter of NanoWriMo.
Karin Hacker clued me in, back in 1999, and I've done it every year since. This is indeed my 15th year.
I wrote a lot of fanfiction in the early years.
I first won in 2005, when I wrote Nikolai.
I seldom do it the traditional way. More often, I use it as impetus to finish a bunch of stuff.
Over the years, I have written many things during NaNo:
Peregrine off High Barbary
Gemini Paradox (later Hard Reboot)
Nick and Corban
Corban in Control
Alive on the Inside
One Gift at a Time
Showdown at Yellowstone River
Power in the Blood
Terror of the Frozen North
Songs for Guitar and French harp
Under the Skirts of Sherwood
An unfinished seriel
That's How We Roll
Angelina Calamity origin story
So yes, for my 15th year, I am doing something. And yes, very much real writing.