There are a lot of scams out there, one way to tell is if they offer market research or patent research RUN!
I make prototype and production plastic parts and I work with a lot of designers.
That is what you need, a designer. I can refer a few if you like.
Get a nondisclosure agreement first thing. See an attorney or get one offline.
Talk to a designer then get a prototype.
Market your product. The more you do yourself the more money you will save.
Let me know if you have any questions.
You asked for someone who's used/gone with one of these companies, and we didn't (not everything fits into an existing box), but there is one company that I would consider if I was going to go that route ... (and APUC is dead on about doing things yourself; saves money and you really have more fun and pride in what comes out the back end).
This is Chris Hawker's site; the link is to the "story" on his PowerSquid because it's the kind of example you are seeking ... he's done it and writes about it in a very honest and educational way; and he has the kind of company I think you want ... someone who walks the talk and is no b.s. (everyone needs to feed the monkey but he's one of the few of "these guys" I've found that doesn't make me ill~).
Anyway this is his pay-to-play site: http://www.invent-shop.com/ ... good free stuff there too ("The Inventor's Mind" is well worth the read and you only have to cough up an email addy).
Cmugs - check out www.studiomoyadesign.com
We specialize in industrial design and product development with 22+ years working with US and Asia based manufacturers. As a former VP Design and design leader at Timex, Vtech and Office Depot - I have developed products and equipment in many categories for global markets.
If you are interested in further discussing, please give us a call at (888)445-1078
I am a senior mechanical designer and I would tell you to stay away from them. I have chatted with a few of them and they are out to sped the money. You will end up with less than 5% of profit. And profits can be burned. If your products have a high profit margin, you need to keep control, use freinds or family to fabricate, patent and try to start it yourself or with a partner. Never loose controlling interest. If you have moderate profit margin, seek manufacture with similar products and see if you can cut a deal, after you get your patent.
I whole heartly agree with kc57hd. I would not look for product development company. They love to generate paper work and presentation and spread sheet and whole lots of Powerpoints.
You or your partner should be directly involve in making, first, the non-functional mock-up hand build model, then, the functional hand build prototype next. Then, you can use these to demo and show and tell and generate potential customer feedback rightaway.