Is Get Games Go legit?

Yes it's legit. It's run by Eurogamer.

Yup i bought Red Orchestra 2 from them, got the key in 2 minutes.
And also, i thinks it's non-steam.

TotalBiscuit has also endorsed it in his latest Salebox video. I bought FM2011 there earlier, got the key immediately, entered it and had the game in my list... even though I can't even find it in the store. ^^

Mafia 2 is steam-able, it even says so on top.

Its a great site. I bought Serious Sam 3 from there for $12 when it was on sale. They gave me a Steam key as soon as I paid. They also did the indie bundle with Explodemon etc.

Are all keys steam redeemable, or only those marked with Steam.

yeah

I bought two copies of Serious Sam 3 there when it was on sale. Fast delivery, good price. The customer service is very responsive as well.

Yes it is a legit store, it i part of Th Eurogamer Network