yeah my galaxy s runs also smooth with the ics builds, but it took very long to get there, but the biggest issue is with all that custom builds, because they had to reverse engineer the drivers, the battery live sucks. I will never go back again to such an device as long as I can get a nexus, were I have the guarantee that we get the drivers to make a real aosp with good experience and good battery life.
Its simple, on the sgs I had to flash I think 1000 times and i was never fully happy, on the nexus I can flash any custom rom and all just works great, and that is since the nexus is on the market, so I bought the nexus and flashed aokp because I like the features it has and stay with it since then.
About HTC great build quality I can only say pffffft no thanks, i had 2 HTC phones in my life, on both the build quality sucked, if I read now the one x reviews, then I think it is still the same, it is not good. Even my Galaxy S looks after 1 1/2 year better then my HTC phones looked after 4 months.
But if you like the one x then get it and enjoy it, I think it is your decision and it need to be good for you and not for anyone else.