To be fair, I have not had the opportunity to play on a GC V. However, I have played on enough GC's throughout my 15 years as a pool player to know how the other generation GC's compare to the Diamond pro/pro AM and I can emphatically say that the Diamond has the better product for sure.
Seriously, look me in the eye and tell me that the Brunswick GC's laminate rails will hold up better than Diamond's Dymondwood rails or even hardwood? Not a chance. Having had a close look at the construction of each, there is no doubt that Diamond's table will be with me until my ultimate demise and playable even longer. I can't honestly say that about the GC's.
In the end, folks just have to make up their own mind. For some, it's all about styling. For me, it's about solid construction and playability. When and if I encounter a better built table that plays better than a Diamond, then I'll readily admit that and even go buy one, as I am fastidious in all matters concerning pocket billiards.
Also, for what it's worth, I play regularly with two people who have now traded their Brunswicks for Diamond Pro tables. Both are avid players. What does that tell you? About the GC 1, it's ok, I'll admit that. I play with another player who has one of these (it has been restored) in his basement. He likes my Diamond more.
In the end, I think the DeBeer's family has it right: Diamonds are forever.