from what i've read he's fairly hit and miss, due to being ratehr formulaic, however i don;t find him a particularly bad author. over the last 15+ years or so i think i've read pretty much all of his 3 book series. Some are alot better than others, though it's like digestive biscuits. not particularly exciting, but if you are expecting a digestive biscuit, it's exactly what you are going to get.
i've just picked up the first of his new series that's a bit of a techno-magical post apocalyptic pre-new world crosover, that seems to be at least nodding towards his non-shannara books.
definately a digestive so far, luckily i was after one.
the Wishsong series is by far (IMO
) his best work if you are interested in trying them out. though it's probaly pretty dated now. I'd agree 'sword' is piss poor, but was his first stuff over 20 years ago. i'd say in general he's beter than piss poor. certainly better than david eddings....