Depends what you want out of your stay. If you just want somewhere to sleep before you fly out of Heathrow/travel onwards in UK, and don't mind unfriendly service and crap, expensive food, then they are ok. Clean and comfortable. But IME (all business travel rather than holiday) you are stuck in teh hotel, they know that and don't give a wotsit about good service. I now prefer to stay in central London and take the Heathrow Express train out to the aiport. The train isn't cheap but think you would probably make most of that back by getting cheaper hotel in London and being able to go out to eat/go to a supermarket for food. That said, it is a fair number of years since I ditched Heathrow hotels, so others might have had better experiences more recently.
Thanks llm. As stated before, it is just for one night, as a stopover, and we really only what somewhere clean, preferably with an indoor pool, and a buffet style breakfast the following morning to set us up for a long day.
Personally I would choose the Novotel over the Holiday Inn, although I wouldn't stay in either of them.
If you can do without the pool, try the Radisson or Arora if you can get a good deal (just looked at the Arora - £63 for a double room and breakfast tonight, and it is actually not a bad hotel), or if you just want somewhere basic you could do worse than the Park Inn.
All of this assumes you are departing terminal 1/2/3 in the morning. If it is T4 the convenience of the Hilton takes some beating (you walk under cover from hotel reception to check in).
Remember though that all of these hotels suffer from huge demand (which makes them expensive) and competition for good staff (which makes service poor) so don't expect good value anywhere.