Agree with TacNuke that CVWs aren't the same as being a plebe. That said, I think it's important to take every opportunity to experience a SA you're thinking of attending.
If cost is prohibitive, that is clearly an issue. But if you can manage costs, I would recommend it. I don't believe mids write evals on their drags. Rather, the event is designed to help candidates decide if USNA is right for them.
If you didn't do NASS, I definitely recommend CVW. If you did NASS, it's still useful (as the entire brigade is there). I suggest asking for an early date for 2 reasons. First, the earlier in the year you go, the more you'll see what plebe summer & early plebe year are like. By spring, things are usually more relaxed. Second, you're prob making your decisions now -- thus, seeing USNA now is better.
Admissions-wise, attending CVW makes no difference. It really is offered for your benefit.