I've had and still have some in planted tanks. They do graze around and will eat smaller fry. Also, they will steal food from fish if they can get their hands on it before the fish do.
The only time i've noticed ghost shrimp to eat adult sized fish are in confined spaces... and if they are really hungry... i've been lazy and put them in a bag with guppies when i got some from the LFS, and it did grab a good sized one and attack it... never again will i do that.. hahaha..
They might seem like a cheaper alternative if you don't want to spend $2 bucks for an amano, but like liam said.. since they are manily kept as feeders at the LFS, they don't do so well in transition from supposedly being a feeder getting eaten before it dies.. to a shrimp you'd rather see swimming around. This is also another reason not to get ghost shrimp if you are using them as tester shrimp for a new tank setup, b/c if it dies does not mean the tank isn't ready for shrimp.. it may have just died in a mature tank as well.
If you like the clear look of the ghost shrimp.. amanos are definitely hardier shrimp and more of a cleaner shrimp, and i've never seen them eat any small adult fish.