I have also bought and I'm using the Rebel SIM. I paid £30 for a special offer and they sent me 3 very thin wafer-like sims, with a tiny processor on them. I also recd a special hole punch with a template for cutting a hole in your mobile phone SIM. This is so that the tiny processor can fit through the hole. They also send a programmable SIM reader so that you can update the SIMs they send you if the "unlock code" in the chip stops working due to an Apple update.
Finally they sent three Pay as you go SIMs, I guess to practice cutting holes in, but it was so simple they were unnecessary.
I cut the hole in my Tmobile SIM, place the Rebel SIM on top of it and inserted it back in the Iphone 3G and presto hey it works. Thats all there is to it.
Ok to be honest there was a bit more, where I specified in the phone settings what data network to connect to and what the username and password were, they are easy to find on the internet. This was only necessary for the internet/data stuff, the phone was able to make calls and receive calls without that. And it does display the Tmobile 3G symbol.
You should be aware of a few things:
1. When you try to sync with iTunes, it knows that you have an invalid SIM, but it won't stop you from syncing.
2. You can't connect to the app store. If you want to, then borrow an O2 SIM download the apps you want then put your own SIM back in.
3. Be patient!!! I turned the iphone on and off loads of time to try to get it to pick up the Tmobile network, in retrospect I should have just waited longer.
4. My wife used the phone whilst she was in South Africa and it just picked up Tmob partner out there, she didn't change any settings on her phone. She did turn off the data services! Otherwise I think it still connects to the UK data network and you get a massive bill.
5. I did use the dev team 2.1 jailbreak to pwn the phone. Not sure if this really made any difference to anything.
6. I used the Rebel SIM in my old 2G iphone and it worked fine there too.
7. It also worked in a Tmobile Vario2.
So all in all worth the money, in my opinion.