The food was undeniably delicious - the steaks were cooked perfectly and the side dishes and desserts were very nice. Of course, for the prices you're paying you'd expect nothing less.
However, the service that Gaucho seems so renowned for was rather lacklustre - there were no actual problems with our order but the waiter seemed bored and just wanted to get to the end of his shift.
The main reason that I didn't take to Gaucho very much was that I was there with my Argentine girlfriend, and I'm a big fan of Argentine food and culture. Gaucho didn't feel authentic at all - some of the most typical cuts of beef (asado de tira and vacio) were not on the menu, nor were there any blood sausages, sweetbreads etc. None of the staff seemed to be Argentine.
It's a nice (if expensive) steak house with an Argentine theme, but not an Argentine restaurant. Personally I prefer going to one of the several smaller Argentine restaurants in London, even with all the little quirks they tend to have...