I should say that my wife and I chose basic Arkham as our "gateway" for boardgames based largely on the theme and the promise of coop gameplay. We were extremely pleased with the results, despite what many people cite as serious flaws of the game...its length, the rules complexity, the length of time it took to set up and put away, etc. If you are one of those people, Dunwich will likely not address any of your concerns. It takes a long, complicated game and gives you even more options, capable of being added incrementally or all at once.
What it does do, even implemented piecemeal, is significantly alter the game balance, in our opinion for the better. We tried it in "touring mode" first where you put all of the Dunwich cards on top of their decks to maximize the experience, and while we did ultimately get crushed in that session, it was easily one of the top plays we've had of the game. Since then it's become a fully integrated part of our routine, because we find what most of what it adds indispensable now and after one or two plays, really not much more to keep track of if you could handle the original.
Its strengths: new encounters, new characters, new mythos, new old ones, new monsters, new items and allies. I look up on those as a real improvement to the game that I can unequivocally recommend to someone that already likes the base set. The game does get harder, but the experience overall is a much higher quality one, if that makes sense. I never got to the point where I was seeing a lot of repetition with the core set, but this virtually guarantees that won't be an issue for a very long time.
Its less well rounded additions: the additional board, the Horror itself, the vortex mechanic. I like these but I can't say they work as smoothly as I'd like, at least within the context of our mostly two player games. I haven't dropped them from gameplay as we've continued, but I could see myself phasing out the board and those two parts in the future, especially when the upcoming Kingsport Horror is released. I will still be pleased to have the rest of its additions and consider them well worth the price of admission.