I've been contracting with HK for a while now and their systems are solid - it's quite easy to make sure that you get your pay and expenses processed. If you need an advocate, the team is happy to do that for you and they're responsive to outreach which is really nice. They also have a partnership with Flare which is nice to have.
Cons
The agency does require you to advocate for things such as rate increases to better align with in market rates. Even when my contract was up for renewal, I had to push for an increase