Moo has excellent quality products. We've used them for business cards, brochures, and several other pieces for various clients. We especially love their Luxe line of products, they have such a great weight and paper texture, and we get compliments on them all the time. They are a little more pricey than companies like Vistaprint, but we think the above-and-beyond quality and customer service are worth the price.
There's not much to dislike, other than it being a little more money than other online printing vendors. But again, the quality is worth the price.
If you are considering using MOO, I'd definitely say get a small run of one of their products. It's completely worth the price, they've got great customer service, and I've had nothing but good experiences with them.
We use MOO for several of our clients' printing needs, and have had great experiences every time we order.There was actually an instance where we had cards sent to the wrong place, and we contacted their customer service and they had a fresh batch sent to the correct address free of charge. Just great customer service, and a great company.