Well, good and bad are very situational: I'm certainly not saying it can't work.
The overall trouble with short-run plastics manufacturing is simply that people don't expect to pay a lot for small plastic things. Unless you've got a really sharp niche, you're competing with people doing runs of 100K from injection molds in China, and they set the expectation for pricing. How much "should" a 2" plastic thing retail for? A trip to the dollar store tells you, $1, $2 at most.
That the other responder in this thread suggested pewter is telling...the thingis that once you're into hand casting -- paying someone to make molds and then pour, demold and chase each individual part -- you might as well use a better material than plastic, because it's going to be just as much work and you can charge more for the result. (This assumes that your part is readily castable, which if you didn't specifically design it to be is not immediately likely.)
I don't have specific experience with Protolabs. I agree on "be selective": get lots of quotes but save your money for the very best fit.
I'd also say, research your target method intensively using the Internets, and do your best to make your design manufacturable -- castable if you want to cast it, paintable if you want to paint it, etc. -- before sending it out. If these guys sense that you don't understand their process and won't make your design sympathetic to it, they'll quote high to get rid of you.