Got to add my two pennies worth...
I had a good few Golf GTIs over the years and was almost definitely going to get a 3 door Mk 5 in either white or red, anyway my situation changed and I needed a 5 door (the wife was pregnant

).
So I test drove the Leon FR and Golf GTI back to back for at least an hour in each over varied surfaces here are my thoughts:
To drive they both drove exactly the same, I know the reviews reckon there are subtle differences but not so much that I could tell.
The interior of the golf as everyone's said is far better than the Leon.
The boot in the Golf is bigger but I can get the pram shopping etc so no problems (I was coming from a Mini so every other cars boot looks massive).
The Leon I think looks great with the optional 18s and in a light colour and get quite a lot of +ve comments about the looks (as many as with my fully loaded, kitted Mini).
In the end I went for the Leon because it was a change from VW, its rareness factor, good looks and I got it fully loaded with everything (18s, bluetooth, bi-xenons, sunroof, armrest, rear airbags, winter pack, convenience pack) for less than the price of a standard GTI.
Yes it has compromises with the interior and its not got the cache of the "Golf GTI" but thats about it as far as I can see. I went for it knowing this and don't regret it.