We stayed at Pop for a week in May (room only) which I booked through the US site as it was nearly £100 cheaper than the UK site.
I didn't have any problems whatsoever and found it easy as to make ADR's ect. because I already had a direct Disney US reservation number (rather than the UK one). The only thing I would say is you don't typically pay upfront in the US, this is usually done at check-in (if you're booking room only) but I simply phoned up and paid over the phone in advance
With the deals on Disney.co.uk recently we have stuck with them but booked on .com in the past no problems. Main difference is on US site you only pay 1 nights deposit and balance when you get there, with UK site you pay upfront.
I use DU for cruises but again no problems at all, excellent conmpany.
We booked with US disney last year as it had the deal I wanted at the time we booked.
No problem at all - did it online and paid the balance before we travelled. The documents were handed to us on check in rather than posted eg free mini golf voucher.
Got a value resort with free qsdp which we upgraded to DDP. Advantage of US offers is you only need a 2 day ticket to get free dining (was a one day ticket when we did it) We already had tickets so better for us.
US deals are not usually released as far in advance as UK.