Very Good Stone Owners

I'm helping a friend shop for his engagement ring stone, having recently gone through the process myself.  When I was looking, I was obsessed with finding the best cut possible.  I opted for an AGS 000 which scored a 1.6 on the HCA.  I thought by keeping it with these parameters it was fairly safe that I'd find a stone that not only looked great in the jewelry store, but also great at home, outside, wherever.

My friend has recently found a stone that he likes very much.  It's a GIA "very good" stone and rates a 3.6 on the HCA.  He's getting a good price for this stone and the size is good for the money.  I've looked at this stone and compared it to a GIA excellent side-by-side.  I really couldn't tell much difference.  My friend feels that once the stone is set, the only thing that'll really stand out is the size.  I agree with him on some level and don't want to rain on his parade.  But I also don't want him to regret his purchase later.  I also feel like the stone could be very good performer and it looked beautiful in the store, even compared with stones with "better" numbers.  Also, I looked at the stone under an ideal scope and it showed the hearts and arrows pattern.

My question is for those who have or have seen "very good" stones, what would be your advice?  Have you been happy with your stones?  Are there very good performers in this group?  I imagine there must be.  How noticeable is the difference.  Having only looked at AGS ideal or GIA ex during my search, I don't really know how to advise my friend. 

Thanks in advance for any thoughts.