OK, so the phone itself is probably brilliant, but and there is a big but, the advert is misleading. I bought one of these as a Christmas present for my son, it was advertised as unlocked, bought as unlocked but received as locked to Vodaphone. It shows up in my purchase history as unlocked. At no point during the purchase process was there any indication that the phone was locked to a network. This caused an amount of consternation over the Christmas period, numerous phone calls and a vast amount of general annoyance and disappointment.
The vendor claimed that it was marked as locked to Vodaphone, but this information only seems to be visible when looking at the "other buying options" feature and certainly doesn't appear on the order history. In the order history the phone is displayed as being Unlocked. Looking at the other buying options now seems to indicate that not all the phones available are unlocked, which is misleading. I can't fault the vendor for their post sales support and help but in the end the phone had to go back and I lost an amount of money on the deal (no full refund because the phone was opened and had been used, albeit just enough to see that it didn't work).
In the end, if you want an unlocked iPhone 4S buy it straight from Apple for around £500 for the 16Gb model.