I stayed in the two bedroom Opera Suite at the Sofitel London Heathrow Hotel in November 2013. It's an amazing hotel, with excellent food and extremely modern and comfortable rooms. And I didn't even know one could get a hotel so clean, as I didn't see one speck of dust in the entire place!
The proximity to Terminal 5 at Heathrow via sky bridge (i.e., walking) was very important to me and my guest, and overall, we were thrilled with the hotel choice. The service is excellent, and you will not be disappointed at the numerous high-end amenities (spa, salon, restaurants, etc.).
A caution to budget travelers: overnight at this Heathrow hotel is extremely expensive (but on par with London standards). Access to Terminal 5 via walking doesn't come cheap, but for me, it was the absolute perfect end to a luxurious trip. FYI, they also have day room rates if you only need a brief rest and shower.