I think your chances depend on lots of factors: crowd levels, number of reservations booked that day, number of no shows that day, the mood of the CMs, etc.
Overall, your chances at lunch for H&V are much better than your chances for dinner at 'Ohana, for instance. I'd show up at the beginning and/or at the end of lunch and inquire about a table. Seems that folks around the DIS have had some luck with walk-ups at those times rather than the middle of a meal service.
If H&V lunch is really important to you, stalk the online ADR system and the "cancelled" thread for your month on this board. I have been very lucky getting what I want for our upcoming trip that way.