Hi, I'm new here so go easy on me!
I've only ever worked for Prospect Solution, but I have worked for them for 5 years so I can list their good and bad points:
Good: They pay reasonably well compared to other freelance work I have checked out.
They pay promptly every time.
Thier instructions are usually really clear and easy to follow.
They may list loads of fines, but if you follow the brief and submit the work on time you will be fine; I have never been fined once.
Bad: They promise a much higher income than is possible. Eg. If you have a brief due in 5 days, you cannot complete any other work during this time and usually during the following 7 day amendment period, limiting the amount you can earn.
There is also a limited amount of work available. I am able to write in two popular subject areas (eng lit. and education) and during 5 years have only had 17 jobs.
Sometimes they answer queries promptly, sometimes they can take days or never answer them at all.
They are a genuine company; just take what they offer with a pinch of salt. There really isn't much chance of a substantial income, but I have earned nice money for holidays etc from it.
I work in the UK, I don't know if this makes things any different?
Hope this helps