I've done a lot of work for inventors who use similar services (although I've not had any feedback on this particular group).
My advice is to be very careful. You take all the risk--they take none. You risk losing your idea and perhaps losing plenty of money. Generally, if you need to pay anything up front, look out for a rip off. Not that no services are provided, but that the services are directly useful for nothing.
For example, I once had a client that forked over $15,000 and an extended period of time for some "invention consulting". What they got in return were some inconclusive focus-group-like studies, some mediocre concept drawings (obviously not done by industrial designers), and a steep bill. No prototypes, no patent, no tooling--nothing of worth. Many of these companies don't know what it truly takes to get a product on a shelf and keep it there.
If you go with someone offering invention services, make sure you get something tangible as part of the contract and that you pay nothing, except out of income earned by the final product on the shelf. That way, you'll have a patent, a developed product, and income--from which you'll pay the invention helpers after they've done their work.
I've been able to get about 30 product designs on the shelf working with a local invention/development house who uses the principles above when working with "outside" inventors. The inventor pays nothing unless the product makes something of itself, so the risk goes to the particular development house and not the inventor. Look around--many reputable companies aren't so flashy--they just do their work well.
Jeff Mowry
Industrial Designhaus, LLC