As far as I know, the only difference is that "VIP" appears next to your reservation at the restaurant. What the restaurant does with that tid-bit is their call. I always qualify, but I can't put my finger on anything positive coming from it. I suppose the best you could hope for is that, given a busy night with an unanticipated wait, you may get seated earlier rather than later.

IDine, OTOH, gives VIPs bonus miles. ^