Can't answer for p2p/torrents/etc. (don't use them) but for regular downloads and Usenet I have noticed the odd peak-time slow down.
It isn't much (maybe a 10-20% loss of speed) and doesn't usually last long (maybe half an hour, or so) before speeds go right back up to around 37Mbps, but it's not a particularly welcome sign (given our exchange has only been enabled for five weeks, there surely can't be local congestion already).
Hoping BT actually increase capacity as time goes on (as an 18 month contract is a long time).
For me; it's great (as I came from a 4.5Mbps ADSL2+ connection, so Infinity would have to be really useless to be slower than that).
As for the modem location; BT will install it on an
extension (they can just make that socket the new master) but I don't think the engineer will want to push the signal round every socket in the house (so you can pick and choose the modem location at random).
They tend to want to fit a new master socket face-plate and that needs to be the single location of your modem.
vDSL2 FTTC Infinity with BT
DL Sync 40Mbps
UL Sync 10Mbps