I had my number transfer from T-Mobile go through today and so went to work with both phones.
First thing I noticed that something was happening was at about 1030 when I wasn't able to send a text from my T-Mobile number. I tried calling my number from a landline, and my T-Mobile number rang. Turned both my phones off and then turned them on at 1100. My iPhone said no service so they went off again. At 1130 I turned on the iPhone and had service. Sent a text to a friend and called myself and confirmed my number had ported successfully. Weirdly, my T-Mobile phone was still making calls, and still showing up as having the ported number in called ID.
When I checked T-Mobile phone a bit later, it was indeed dead. Transfer was nice and straightforward... although I was told on Monday it would happen on Wednesday only to receive a text on Wednesday saying it would happen on Thursday.