I have not used a US agent before but have talked to people and read about peoples experience.
Things to note,
You will be dealing with the US and the US part of the cruise line so you need to consider call costs,(some agents have uk 0800 No. and there is VOIP to keep costs down)
ABTA,ATOL protection does not count,
Your deposit will be refundable up to final payment.
You should be able to get any price reductions.
You will probably be cruise only so need to sort out flights/transfers.
Your UK travel insurance may not cover a missed departure.
IF you get the cruise line confirmation No. then the cruise line will probably honour your cruise if the TA goes bust(same as the UK)
YOU CAN get better deal but not always.
The two cruises I am looking at for this year are at least £100pp cheaper door to door. and there was a Navigator 14night deal for 30Oct under £600 which was £200pp cheaper than the best UK price.
I just got an email(uk agent) for the NCL jewel 7n July UK with flights £599 now that will be hard to better because the DIY flight prices are a lot higher in july.