Probably not worth it. There have been a few threads on it. Essentially you will probably have two lots of NI (as opposed to one) and possibly an umbrella company fee coming out of the rate given. The rate given might also include holiday pay, so you wouldn’t get anything extra for holidays if that is the case. The argument proffered for might be that you can submit expenses to save tax, but in all likelihood, you would be worse off overall.
For some reason they difference in pay rates is quite often £1, which actually saves the agency money.
That pretty much sums it up.