I have a Panasonic Lumix GF3 with the stock 14-42 lens. I've been away from photography for some time now. I bought a Raynox DRC 250 macro and was wondering if I would achieve pretty good results with the stock GF3 lens or do I need a telephoto-style lens to get decent macro shots?
With a macro converter like the DCR 250 you are going to be trying to balance between getting a workable depth of field (creatively interesting, but challenging) with longer lenses, and getting rid of vignetting when shooting wide.
I've found similar setups require a lot of care in setting up, but your stock lens should, especially when racked out, be able capable of capturing solid images.
Here are a few examples of images taken (not by myself) with similar setup to yours (all at 42mm) to give you an idea of what is possible: