My wife & I had a very nice dinner there this past Saturday. Yes, it is a now a corporate/chain operation, but the food and service were more than acceptable.
I had the bruschetta w/ tomatoes & arugula and the penne w/sausage. My wife had the gnocchi w/spinach & gorgonzola and a side salad of tomatoe & red onion. The portions of pasta were large and could easily do for two people. We shared a tasty panna cotta for desert.
Carluccio's is located on the 3rd level of Terminal 5, before airport security. I am not claiming it is worth a special trip from the other terminals at Heathrow, but it is certainly a worthwhile option.