I've had some basic work done here with good results: hemming dress pants, hemming jeans, taking in a silk blouse on the sides and sleeves, tacking a low-cut blouse. All work done was perfectly fine, the price was reasonable, and the work was completed quickly.
I would recommend them for basic alterations, but not for more complicated ones. The reason I say this is that the person measuring you/pinning your garment is not the same person that does the actual sewing/construction. The person pinning you at Alterations Express is less knowledgeable about garment construction and the limitations/possibilities of the alteration. For example, a woman pinning my pant legs to make them slimmer had to be told to pin them from both sides on each leg because they had a permanent crease... Also, the process is a little rushed and you have to know exactly what you want. A more professional tailor would consult with you ahead of time to discuss options, perform the work, and work with you to make any additional adjustments; this would be the same person for all of these activities.