When you look at it T-Mobile's £29 monthly fee is not a world away from fixed line broadband. The latter is going to be £10 for the BT line and something around £15 for the ADSL connection. So for another £4 you're getting total mobility chucked in - and that could soon be made up when compared against typical landline usage, break even might not be too far off.
I know that there are now "free" deals on the table from the likes of Sky - but the comparison is still very attractive taking into consideration all of the variables.
T-Mobile done good