They're not the worst in my experience. I did a short stint with them on two contracts, one was extended from 1 to 3 months. Was paid properly as agreed and on time.
Main thing is your own abilities, some of the full time crew I was contracted to fly with, who weren't Acass contractors, were quite useless. On other occasions those full time crews may be very wary of you as they will mention having had poorly skilled pilots coming from Acass before.
Just be mindful if the position looks like it could lead into a full time role, then I'm sure Acass will want to stay as your employer and keep a nice percentage of your possible earnings!
Best of luck,
S.C