Had it for a couple of months and yes you are correct re: the speed. I originally tested 19mb on the website, in reality though, my thoroughput was set at 46 down and 15 up, although this has been changed to 38.7 down and 10 up as the service doesn't officially take higher speeds yet. They anticipate it should go to 60 down in the not too distant future, I'm also eagerly awaiting FTTH that's currently being trialled in Milton Keynes which is 100 down.
Unlike Virgin, Infinity is uncontended back to the exchange from the cab, so even on peak times the speed drops to 32ish down.
The unofficiai download limit of 100gb is still in place, but I've still yet to hit it, and even so, the speed is only restricted at peak to 2mb and other times runs at full, so it won't be too much of burden.
With regards to BT's money, BT made a loss when they sold Cellnet, the company took the hit on the loss of the 3g licences that were bought and sold the company debt free to mm02, which later became 02 under the ownership of telefonica.
As far as I can tell the higher speeds that Virgin are trialling involve using bonded channels so they're 100mb service is 2x50mb, 200 is 4x50 etc, so in the long run I believe that BT should come out in top as the service is shared at the exchange and not the cab which virgin have no control over, time will tell!!