I used to swear by it. If you search, I'm sure you'll find posts in the past where I've raved about it.
I'm more conservative in my opinion about it nowadays. Yes, it makes the engine sound slightly sweeter and it produces less smoke. And I thought it made it pull better - but I now think I was mistaken.
Trouble is, these engines love to be given a thrashing and hate to be driven gently. I find that a few days of being stuck in traffic jams makes the engine feel slightly less responsive, and just one or two full-throttle runs up to high revs restores the proper feel of the engine. I now believe that the perceived performance improvement when I put BP Ultimate in my tank was entirely down to this effect and not the fuel at all - after filling up I'd naturally want to see if there was an improvement, so I'd do a few hard full throttle runs to see if there was a difference. Sure enough, there was a difference after I'd done that, but only because the engine loves to be treated like that!
Nowadays, I'm back to ordinary fuel (Shell, because it's convenient), with Millers Diesel Sport 4. That combination is cheaper than BP Ultimate, and it is at least as good.