I'm a big Terry Brooks fan, and have been since 7th grade when I stumbled upon one of his books. The original trilogy is actually low on the list of my favorite "sagas", but the key there is that the first 3 books can be read stand-alone, with no knowledge of the ones before them, not to mention they were his first published novels (and they were written 30 years ago). It's not until the Heritage of Shannara saga that things become intertwined and, IMO, much more enjoyable.
I won't dispute that "Sword of Shannara" is kind of a Tolkein rip off, but I personally enjoy it more because Terry's style is different, IMO (where Tolkein would give you a dozen pages of back story, Terry gives enough to keep you interested but still leaves plenty open to the imagination). He just fits my preferences better, so take that comment for what it's worth.
For me, it's more about the characters than the quest they're on, though once you get to the "Voyage of the Jerle Shannara" trilogy, the quest itself was definitely one of the best. You get wrapped up in the characters; their beliefs, their struggles, their triumphs, and want to see how it turns out for them, and Terry delivers there. Yes, there are "typical" plots in many of the novels, but most of them at least have a twist or three you won't necessarily see coming a mile away.
I've read these books numerous times, and am halfway through "Scions of Shannara" as I try to go through the entire series in preparation for the new book in August, "Measure of the Magic", but to be honest I don't think I can really explain why I love them so much. I think it's something you need to just try for yourself and see if you get into it.
Since you have "Wishsong", and the original trilogy books can, again, be read on their own, I say go ahead and dive in. I just finished it three days ago actually, and I can tell you that it explains the essentials of what you need to know to go with the story pretty quickly, so you don't have to worry if you haven't read "Sword" or "Elfstones".
So give it a go and, if you enjoy it, I would say pick up "Elfstones" next and then get around to "Sword". After that I'd read the series in published order. Happy reading, and I truly hope you can find as much enjoyment as I do!