The only review of the Surface Pro 3 that matters - as always, from AnandTech. They conclude:
Surface Pro 3 is easily the best design Microsoft has put forward. If you were intrigued by the previous designs, this is the first one that should really tempt you over. I was a fan of the original Surface Pro, and with Surface Pro 3 I think Microsoft has taken the hardware much closer to perfection. At this point the design needs more help on the software side than hardware, which is saying a lot for the Surface Pro hardware team. Personally I'd still rather carry a good notebook and a lightweight tablet, but if you are looking for a single device this is literally the only thing on the market that's worth considering. I don't know how big the professional productivity tablet market is, but it's a space that Microsoft seems to have almost exclusive reign over with its Surface line. With its latest iteration, Microsoft is serving that market better than ever.