Unless something has changed, the catch comes when you try to transfer money from the US to the UK. Since the US has nothing like BACS, you have to pay for the money to be wired from your US account to XE trade.
That is not entirely true. I just completed my first trade from a US account into my UK HSBC account and it went like this...
I setup my US account as an EFT bank account on XE trade.
I setup my UK HSBC account as a Wire Beneficiary on XE trade (since you cannot EFT into a UK account from XEtrade).
26th June - Initiated a transfer from my US account to my UK account. (EFT -> Wire, there is a small fee for doing a wire but it's the only way to get the money to UK accounts. The fee is way less than a bank would charge).
26th June, End of business day - Received an email from XEtrade that my funds had been withdrawn. Confirmed this online with my US bank.
27th June - XEtrade status is "Processing" meaning they have received my funds and are processing the trade.
28th June - XEtrade status is "Processing"
29th June, early am - Received an email from XEtrade stating the trade was complete and I should receive my funds within 3 days. Checked my HSBC account online and the funds have already been deposited successfully.
The entire transaction took 3 business days and cost much less than if I had to pay a wire fee through either bank.
Just to clarify, yes I was able to have the funds withdrawn via EFT from my US bank account. But, EFT is not available as a deposit method on the UK side. The only options are Wire or Bank Draft (which takes longer).
I am extremely happy with the whole process though. It was so easy setting everything up. One other thing to note... when you initially setup your US account for EFT, XEtrade does require you to verify the account details by faxing or emailing a form to them. But this is a one time setup and doesn't take very long to fill out.