mtacchi - well here are the reasons I think are against the Sheraton Heathrow:

1. Mediocre rooms, though undergoing rennovations
2. The hassle of getting your luggage to and from your room, particularly when you are allocated a room just about as far from the elevator possible, which the check-in agent said were the only rooms available as the rooms close to the elevator are reserved quite far in advance (so my Gold status didn't cut any mustard there!). Oh try getting a bellhop to assist, good luck if you can find one.
3. Rates are high, given that you could get equal or better rates in the city for equivalent or less

Now why I think staying at any Heathrow hotel is misguided. When you take into account the cost and time taken to get from the hotel to the airport, whether you take a cab or the "hotel hoppa" bus, you've got a loosing proposition. It took nearly a half hour for the bus to complete all it's pickups before I was dropped of at the terminal and I paid 4 quid (I think) for the ride. Cost and convenience are the issue. You could probably get a decent room in the city plus the cost of Heathrow Express for equal or less than what you are paying at the airport for lodging and transportation. The location may seem convenient but I would venture that taking the Heathrow Express is a far more comfortable way to get to the airport and could be just as convenient time wise, maybe better.