Now, careful with CWS chaps...
On the F50 it's a means of re-datuming the FD, and provided that you have armed the altitude capture mode, you will still level off where you should.
In the Boeing, it takes away some workload, and thus reduces arousal. So, you're climbing or descending to cleared level, in CWS. As you approach the cleared level, you'll get all the usual indications (800ft to level chime/'c' chord) and light, but the aircraft will sail through the level unless you do something. There's a danger that you won't do anything because of the relatively low arousal. You'll know about this when you get the altitude alert passing 200/300ft the wrong side of the level. By then, of course, it's too late.
So, I don't use CWS other than in significant turbulence or if hand-flying and I need to put a chart away quickly or whatever. In the latter circumstances, I never leave it in longer than a few seconds. It's also handy when you've been hand-flying with raw data, and you're about to put the automatics back in, because engaging CWS legitimises use of the MCP without having to call for everything.