Just did it two weeks ago. The sim you'll use for FTI is the same you'll use for the interview (same location at United training center), and instructors are either retired or current United instructors.
Know the callouts and profiles cold before showing up for prep which will help immensely, especially on the go around. You'll have two hours in the sim to just go over all the profiles and basic airwork with the 733, with additional time for brief/debrief.
The whole experience, from FTI to everyone in the new hire office, was light years beyond what I expected. Everyone wants you to be there in a future class and treats you as such as long as you're willing to reciprocate!

And i got the job. Sim prep was worth its weight in a class date!
I hope this helps!