They're very good, in fact, the more recent ones have been extremely good.

But my advice is to avoid the ones by Keith DeCandido. He's an arrogant ass who claims to be a true fan of the show, yet consistently gets the characterizations of the boys totally wrong. He even gets simple things like their eye color wrong and used cringe-worthy nicknames for Dean. Ugh. I could go on and on, but that's just my 2 cents.

All the other novels by other authors are quite good. :)