*UPDATE* Changed a few things. Renders come out much crisper with these settings. Give em a try.
I've changed my settings since this time. Try this...
But yeah shadows can be a problem. I usually make my own in photoshop, but if you are using a full environment background set with ground the issue of shadows becomes a "lighting" problem. Lights make or break an image in my opinion. I've shopped around looking for good ones. Renderosity has some great ones for cheap. But it all really depends what you are going for with your picture.
Here are a couple tutorials I've used to make my own lights:
Hope this helps.