I've had a peanut butter beer and a couple of hazelnut flavor/extract-added beers; all them were drinkable, but none of them were exactly triumphs of the brewer's art. Probably the best I've had is the Rogue Hazelnut Brown.
If you're brewing with actual nuts - peanuts or tree nuts - it's a royal pain to get the oils out.
If you're adding extracts, it's tough to get the balance right so that the beer isn't overwhelmed by the flavor. Also, many nut flavors are very artificial tasting, so you've got to keep it subtle.
In all cases, nut flavors work best with malt-focused brown or dark beers.