The KDP Select program has now been around for nearly two years, and CJ Lyons has a well-thought post on Jane Friedman's blog:
The Select Program is just that: a program, a tool, a tactic.
In other words, it’s not a promise, a guarantee, or even a long term strategy—for authors or Amazon.
It’s a program. Subject to change. And it might not fit your needs now—or later—or ever. Or it might be the catalyst you need to engage new readers and propel sales.
Some authors have had great success with KDP Select. As with any tool, it depends on what your needs are. And occasionally, even though we're talking about writing, it pays to create some revenue models and experiment with them.
This is an unprecendented time in publishing, where authors are assuming control over their own output, and their own careers. Not every tool is going to suit every author. But it's still worth knowing everything you can about what's available for you.