I´m a member of ONE extra-agency (they also hire for small acting rolls), even though several ones exist in the city.
I picked carefully and I suggest you should do the same. There IS a lot of crap in this world. My agency has their own database, they take the castingphotos of their extras for free and they suggest their extras to companys.
So all I have to do is wait until I get a mail asking me if I have time for a certain movie. They do all the work for me. For free.
However: there are some other agencys in the city that are okay as well and they ask for a small fee, so it is okay if it costs something. As long as it is not much!
Look at the websites carefully.
A good agency should list what kind of work they did in the past. Some current examples for my agency would be Anonymous (btw I was an extra there), The Apparition, Hanna, The Three Musketeer, Unknown, Inglourious Basterds, big german movies and so on. As you can see some big stuff... That is a BIG hint that this agency is obviously a good one.
AsI said my agency is for free in general (they have a special subscription for a tiny amount of money), that also shows that they belong to the better ones. They do not live from YOUR money, but from the money they get for finding extras for big movies.
So yeah, look what they did in the past. Look at the costs. A small amount of money is okay. Do they say what they need the money for? How professionell does their website look in general?
The more open they are to their customers and the more professionell their website looks, the more likely it is they are a good agency and that casting directos really use them.
Making a lot of money with them is of course unlikely. There are millions of people out there who hope for being an actor. When I did Anonymous I was surrounded by many many who had the (not very secret) wish to be discovered by one of the important people running around. But surprise, surprise, that did not happen. The maximum some people got was a single sentence. I had one in Anonymous. So in case they do not cut it out you´ll see me swearing into the camera if you watch that movie once it´s out.
I do believe though that sentence will be cut out. I will not become known with it, even if it stays in the movie. It won´t even give me more roles in the future. It will have zero impact. (Actually I wouldn´t even want that. I wouldn´t want to be an actor.)
But you are realistic with that allready, so telling you that most likely you won´t be discovered that way does not crush your dreams.
BUT it is nice to do that work, it´s interesting, it´s fun and you meet many nice people. Sometimes it can be quite hard, but I always had fun with it. Even when I fell asleep while standing or when I was freezing like crazy because they wanted to do a rainshooting...
Personally I´d suggest everyone who has the time for that to try it out.
So go for it!
I have to say though that the website you posted looks quite weird. I am really missing some more details how they work, what they did in the past and so on.
I´d give you the link to mine, but I doubt it will help you since it´s german and you won´t understand much.... lol
But in case you understand a little german:
agentur-filmgesichter.de
So you have something to compare.