Many people know straight away that NaNoWriMo is not an event that interests them, although this opinion may be subject to change in later years. The rest of us either fall into the 'I definitely will' or 'I might' categories. To aid your decision, I present here some of the things other writers have said about taking part. I've tried to offer a positive, yet balanced, selection of views, as ultimately NaNoWriMo is not for everyone.
My thoughts? I agree with much of what Kyeli Smith says - if you start NaNoWriMo and find yourself hating every minute of it, then you're doing it wrong, or at least for the wrong reasons. It is meant to be fun, but it's also meant to be challenging, so there are times during November when it isn't fun; nonetheless, the good times should outweigh the bad. However, if you plan on making a career out of writing, there will be deadlines and word counts and occasions when (as in all jobs) you don't feel like working, so don't quit just because you have a bad day.
And if you end up wishing you'd never started NaNoWriMo, but know you can write - indeed, have had some success with writing before - then you can give up without feeling that you've failed. Even if you don't make it to 50,000 words, but you truly gave it your best, then who else can judge your achievements but you? Maybe NaNoWriMo is not your thing - maybe writing isn't either. The only way to know for sure is to try it and see.