I've only recently started having items altered, but now I'm sold on the practice. A few months ago I "auditioned" a couple of tailors, and found one I'm very happy with. So far my alterations have all been about making the item fit more closely and more flatteringly, especially coming in at the waist--tops and blazers (or jackets in general), side-seams and darts. And precisely because I buy almost all my clothes at clearance or thrift store prices I'm happy to spend two or three times as much on the alterations.
My favorite alteration items currently are men's silk shirts--the short-sleeved kind made with a tissue-thin but washable silk. (Do a Google images search for "Silk shirts summa" and you'll see what I mean.) I love the feel of them, but I can never find them in small, and even medium is hard to find. My new tailor took one that I looked positively lost in and made it into a slim close-fitting tunic that I absolutely love. (She had to take in the sleeves as well to make it work, but it was still affordable.)
I worry that I might be getting carried away, because now I'd really like to take every top in my closet that hangs loosely in the back and add a couple darts to it. I suppose it would be a bad idea to go for more or less the same silhouette in all my tops.