We upgraded once (using points) to the Butler room and agree that it was nothing special.
It was worth it for us that night because we wanted breakfast in our room before we left for the airport and had items that needed pressing. Having those included in the room made it a value.
Other than that the executive rooms are just as nice (although a few floors lower) and they have the same "UNRIVALLED VIEWS OF LONDON." (which really are very nice)
I love the design of the Park Tower rooms, by the way. It's a very clever use of space that makes the rooms feel much larger than they actually are.
I'm surprised we've never seen a similar design elsewhere.