I work in Sha Tin and love the IKEA cafe on the 3rd floor of Grand Central Plaza. Spacious, good western and scandinavian food and very decent prices.
If you like Cantonese food, especially dimsums, your top choices are the Yum cha joints in New Town Plaza and in City One complex in Siu Lek Yuen. Also the 'floating restaurant' on the Shing Mun river.
People from all over Hong Kong come to Sha Tin for two specialties: chicken congee and pigeon. Theere are two famous restaurants serving both - one in Lung Wah hotel near the KCR station and the other in the lane beside Regal Riverside hotel. You need local Chinese help to get there and order though.
Another lunchtime favourite is Ruby Tuesday in New Town plaza where you can pig out at decent prices, especially if you get their VIP Card which gives you 50% off ( yes you read it right !) every meal