I bought the 3 SIM for the iPad. The download speed is pretty good compared to O2 Micro SIM - 3Mbps vs 0.8Mbps. Upload speed doesn't appear to take advantage of the HSUPA in the area. My pet complaint against O2 at the moment is image compression which I was pleased to find not present on 3. Also there doesn't appear to be a proxy server on 3 so sites like BBC HTML5 iPlayer work properly. The only snag I found is the sign up process - try spelling your name name and address and bank details over the phone to a non UK based call centre - the whole process took 30 minutes.
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