Having worked for ten or so agencies over the years, the two things to watch out for are:
Agencies which don't tell you that you will be paid monthly rather than weekly, in arrears,
Agencies that insist on the contractor providing their own professional indemnity (PI) cover. The cost of this can vary wildly depending on the actual job title the client gives the role, and may make the contract uneconomic for you, particularly if it is a short one. The insurance provider will say that the cover needs to extend considerably beyond the actual contract as problems may not arise until long after you have left. If the job title includes the word 'aircraft' or similar, expect the cost of PI to be about 3X what an IT contractor would pay, whereas the remuneration may be less.