I ordered the SIM at 8.30am yesterday.
It came with the post this morning and it's ready to go already!
So far so good, Three/3!
I'll only be using it for surfing the net and text messages because they are both all you can eat, not for phone calls or anything like that... I'll still be using my O2 number for most stuff, I know and trust them and I've been with them since I was 19 (I'm 39 now) without any problems, so I've now got 3 mobile numbers but only really use 1 for non-internet stuff.
Feel free to send me a test text on the number you've got for me if you want to and I'll reply as soon as I've got it :-)