I echo the same thoughts, within the IT field, you will generally tend to find that you'll get a reasonable rate. There are agency margins - which you can view as the 'cost of doing business'.
My own experience with agencies has been more or less positive. There are a couple of 'dodgy' organisations out there but then there are also the ones that operate professionally and as long as you get a reasonable market rate, let them take their margin.
One thing contractors tend to focus too much on is the rate, or rather every body elses rate. As a contractor youy must make sure you are happy with the deal YOU are being offered. In my experience, it was not unusual for contractors to walk away from a contract over a rate dispute, sometimes over as little as 10 pence in the hourly rate, only to then be unemployed for the next 6 months or be forced to take acontract on a less interesting project. The rate is not the only thing contracts offer. Are you going to be working on a good project? Will you be able to update your skills? Will you get exposure to new skills? Is the location convenient? If you don't like it don't sign the contract, no-one is forcing you. If you think you can do better elsewhere, fine, go and do it.
Anyone thinking of contracting must understand that your contract with the agent is just that, and is between you and the agent and is confidential. It is not the clients business what the agent is paying the contractor, and likewise it is not the contractors business what the client is paying the agent. If the agent is taking a huge margin, when the client discovers this they will not deal with the agent again, the same is true with the contractor, they will not use that agent again.