I used HEX the only time I've flown out of Heathrow. I'd just missed a CrossRail train, the next one wasn't for 25 minutes so I got the Heathrow Express. Expensive, especially given I had a Travelcard, but I was in the airside 'Spoons with a beer in my hand 40 minutes later.
It's definitely a premium option, but it is a good passenger experience. Especially when the alternative would have been a full and standing 345 in the evening peak, and I had luggage.
But is it a good use of paths out of Paddington? That's the debate. I was in the second carriage and had the "third" of the carriage to myself. So I'd say no. But as mentioned above, GWR with their current IET fleet are not in a position to expand services. So if the paths would lie unused then go for it. HAL clearly see value in it. Plus, wedging everyone onto Metro style trains (be that CrossRail or the Piccadilly line) with limited luggage space wouldn't be good for either the airport passengers who are probably jet lagged coming off long haul flights nor the passengers from the likes of Southall and Ealing Broadway who would be left to climb over suitcases just to board the train. Heathrow's airlines and their passengers will be dealing with suitcases en mass, it's very different to the Ryanair crowd who are incentivised to stuff their possessions into the smallest bag you can find.
Most major cities will have a fast train, or a fast coach from the main airport, Heathrow shouldn't be any different. At least HEX actually is a premium service compared to the alternatives, unlike a certain other TOC to another airport who also use 387s.