I have been to this restaurant several times, last time a couple of weeks ago.
They have in my opinion the best "tagliata" in Milano, unlike most other places this is not simply a sliced roasted beef but more exactly a sliced Fiorentina (T-Bone steak); besides this the selection of dishes on the menu is at very good level and the quality never disappointing. If it's your first time there then do yourself a favor and try tagliata :)
Quite good selection of wines too, even though they don't have a long list they include good barbaresco, a good collection of Tuscanian wines, and (mostly) they charge them at a resonable price.
You can have a complete meal for euro 25-35 excluding wine, which in Milano is fair enough. Prices are the same at diner and lunch. Note that in Italy prices ALWAYS include tax and service (tip is optional).
Location is a typical midrange "osteria", don't expect it to be the fashion place to take out a girl for a special occasion or an important customer; this obviously is related to your expectations, the location is perfectly resonable for the price range of the place and the personneel is kind and professional.
Definitively worth it if you want to eat well, if you are having a business lunch/diner with collegues, if you want to go out and are not looking for a "special event diner". On Saturday is practically mandatory to reserve (you won't find a table otherwise).
If you own or manage Ristorante La Cantinetta, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.