I've used it a few times - it's outside though, and a bus ride to whichever terminal which can be a bit of a faff.
If you are going from T5 pod parking can be more convenient, or the short stay which is a 2 min walk to whichever terminal you want. If you pre-book the short stay can be nearly as cheap as the long stay.
If you are travelling from T5, the short stay at T3 or T2 is usually a LOT cheaper, and you can get the heathrow express between terminals for free. T2 is a bit of a walk from the train station, T3 is pretty much upstairs from it, but the parking is busier than T2. Park on a low floor at T2 in a far corner and it's pretty unlikely anyonw will park close to you to ding your car 
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