Sprint works well in Squaw Valley (residential area) and throughout most of the Squaw Valley ski resort. Anywhere inbounds on Squaw Creek, Red Dog, Olympic Lady, KT, Headwall, Palisades, Emigrant, Funitel, and remarkably, Silverado (top-to-bottom) is fine. The bottom of Shirley & Granite is spotty, but once you get about 1/5 of the way up either chair, the coverage is OK. Service can be spotty from the Gold Coast Lodge to the bottom of the Broken Arrow chair, but once you get about halfway up Broken Arrow, service is OK. (I don't actually ski any green or blue chairs and don't spend much time in the terrain park so I can't speak about those areas.)
Sprint has free roaming (at least on my plan but I think on all plans nowadays), which basically means Sprint customers can roam onto Verizon's compatible (CDMA) network. (ATT's & T-Mobile's GSM networks are incompatible with Sprint.) So Sprint customers get to take advantage of the coverage of Sprint and Verizon-- which are probably the best two networks, coverage-wise, at Squaw (if not Tahoe in general). (I think Verizon customers can roam onto Sprint networks too, but I don't know if it's free.)
Based on my research, Sprint has the cheapest plans-- especially if you want unlimited data (which is pretty much the only way Sprint sells data).
I have several friends (Squaw regulars) who loved their iPhones but left AT&T because they couldn't get coverage on the mountain.
Sprint service works well at my house in Tahoe-Donner, and all the way from Squaw to Tahoe-Donner.
Boost Mobile and Virgin Mobile are owned by Sprint so they use Sprint's network (though I don't think they're allowed to roam onto Verizon's network).
If you get an Android phone, there are apps available that let you force roaming onto other networks. Sprint's phones are normally programmed to connect to a Sprint network, if at all possible, even if there is an essentially useless 1-bar Sprint signal, in the middle of 5-bar Verizon coverage. So if you find yourself in spotty Sprint coverage, you can invoke a roaming override app and potentially take advantage of solid Verizon coverage.
There are cool apps available for Android phones, like TiKL (Walkie-talkie), GPS tools that let you see and track altitude, vertical skied, and track your maximum speed, compass bearing, etc. And the speech-to-text on Android phones works amazingly well so you don't have to freeze your fingers typing texts/emails on cold days. You can just push a physcial button with your glove on and dictate: "Text to John" (The text messaging app automatically opens and selects John.) "Great powder on West Face." Glove off. Hit send. Glove on.