Here's a rundown from a longtime local foodie for places to eat. Just remember that this is a ski county and not known for it's food... for that, go to San Francisco or NYC.:
For high end dining: Keystone's Alpenglow Stube is a fun dinner and it's quite good food and service. You ride a couple of gondola's (blankets provided) in the dark to get to the top of their mountain. It's a fixed menu, white table cloth type of place but not snooty (I mean it IS on top of a mountain and all).
They also have a more casual fondue restaurant that I think is called Fondue Chessel or something (sorry for the poor spelling) which is more family style and long tables.
The Blue Spruce Inn in Frisco is good, but it's very much a heavy heavy food place (think big fat steaks), and the atmosphere is definitely mountain casual (wood panel walls, flannel shirts abound).
If you're looking for a good atmosphere and more interesting dishes, try Samplings in Frisco or the Wine Cellar (it's sister) in Breckenridge. But plan on paying a lot of money for not a huge amount of food. Samplings seems to have better, friendlier service. Both restaurants are some of the only places in Summit County that have actually hired interior designers to create restaurant-like atmospheres that feel like they actually though about it rather than renting cheap wooden chairs from a conference center.
Cafe Alpine is an old Breck standard, pretty good although their menu isn't as daring as it used to be. Great wine cellar though and fantastic patio for summer. They also have a tapas bar that is good for early dining or lighter fare.
South Ridge Seafood on Ridge St. in Breck is in my opinion Breck's best food for the value by far. Not all seafood, and a huge menu. Really interesting specials, very friendly and good service. The downside? It's one of the loudest places I've ever been to, and I can't take people there on busy nights. Even on the quiet nights the tin roof ceiling and the owner's reluctance to update the furnishings from the old restaurant to softer, noise-absorbing surfaces is really hurting him. But don't let the cheesy atmosphere or awful wall art fool you: The food is fantastic and so are the prices: Try the Trout, crabcakes, the flatiron steak and the sea scallops. Or the walnut salad, the chocolate ganache for dessert is great.
I'd avoid Relish, the new place that used to be Pierre's on Blue River Plaza in Breckenridge. It's just not worth the very very high prices, and some of their meals are not very exciting nor prepared well. Service is uneven.
Avoid Hearthstone; they have survived only through their really great atmosphere. The bar upstairs is worth late-night cocktails, and quiet.
Mexican: Carlos & Miguels in Frisco opened this year and it's the best Mexican in Summit County, and a real restaurant (Jalapenos is okay for very very casual)
in Breck, Mi Casa is fondly known as "Mi Ketchup Salsa", so that says it all there. I think it's entirely overrated. For decent Mexican in Breckenridge (and you can't go to Frisco), try Fiesta Jalisco, which is merely okay, or Jalapeno's.
Pizza/Burgers: A-train pizza, Frisco is good food but bad service. The pizzas are great.
Eric's in Breckenridge is by far the best sports bar in Breckenridge with an arcade outside for kids that is crazy. Plan on a wait, but their food is basic pizza burgers and consistent.
Empire Burger in Breck has the very best burgers in Summit County, and is much easier to get into than Eric's. Nice atmosphere with TV's, too.
Giampietro's pizza is loved by some, not by others.
Avoid Fatty's pizza.
Sushi: Avoid Wasabi in Breckenridge for sushi--really petulant owner/chef makes you feel very unwelcome, and the sushi is way below par. Instead, if you're in the mood for sushi, go to Flying Fish across the street. Still very expensive but it holds it's own.
There is NO good Chinese in Breckenridge.
Blue River Bistro in Breckenridge is fair to okay. It's very loud and service is unfriendly. Best dishes there are lunch dishes such as Walnut-Chicken salad croissant and the calamari.
Italian in Breck: Giampietro's, a small, casual Italian place in Breck (no reservations), lately has been very very salty, but that is due to a couple of stand-in sous chefs. If you go, make sure Ken or Todd Nelson are in the open kitchen (Todd owns it and Empire), and order the Chicken Marsala off the 'specials' menu; it's their best dish.
Do not eat at Michael's Italian or Taddeo's. The former is good only if you have crabby children who want spaghetti, and the latter is overpriced and loaded with heavy cream alfredos.
Misc in Breckenridge: Quandary Grille is good for apres, and okay for burgers, but not much else.
Avoid Horseshoe II at all costs. Same with Bubba Gump's.
Motherloaded, a new place, is becoming a hot snowboarder bar, and is a fun place and the food is not bad, either. It's a mac and cheese and fried Twinkie place. Can be crowded and very poor service. Prepare to wait.
Rita's, downstairs from BreckBBQ (avoid), has great margaritas, but it's incredibly loud with hip-hop music and the tacos are not great most of the time. The chips, however, are.
Breck Brewery smells like cleaning solution, I have never understood why they don't fix this. Their nachos are great, some of their burgers are quite good and the tater-tots are a fun addition to the menu. Try the Breck Lite beer or the Hefeweisen with a lemon. The outdoor balcony on this place is great in the summer for lunch.
Steak and Rib in Breckenridge is like Blue Spruce: sort of overdone mountain atmosphere but with great steaks and other meat items.
Briar Rose has been around forever; steaks are the best items on their menu.
My Thai is pretty good for Thai dishes. Super casual.
Denzaemon is a noodle place in Breckenridge, it's not worth it at this point.
Mi Zuppa in Breck is great but only open for lunch and closed on Sunday.
Clint's on Main has great bakery items but get ready for really angry service--it's got quite the reputation for it. I'd avoid them for coffee drinks--they serve them with a grimace that makes you wonder if you should apologize for ordering anything other than a cup of coffee. Petal and Bean used to be great for coffee by City Market but has decided to dial back. Cool River or, can't believe I'm saying this, Starbucks are much better choices.
Breakfast: This is an area that is hurting in Breckenridge. Daylight Donuts is a good place if not overly crowded for egg burritos and sausage egg muffins, but do not expect table service or good coffee. Avoid Blue Moose if you don't want crabby wait staff or don't have cash. The food is mildly decent. Prospector is okay for very basics, do not eat their green chile--it's bland as anything.
The best 'big' breakfast in Summit County is Mtn. Lion Cafe north of Silverthorne, period. And I agree about Butterhorn--it's quite good too.
If you're shopping for food to make at your condo or house, go to Alpine Market on Lincoln for natural/organic foods. City Market is best avoided apres ski due to extreme over-crowding, and high prices are the norm. Be prepared to pay them.