I was REALLY surprised OKC decided to trade him this early. They could have waited until the summer to make a decision (seeing as he's a restricted free agent), while using this season to make a run with what the same core as last year. Still, Sam Presti would do this type of thing.
For OKC, it's too early to say whether this is a good deal or not. Martin is definitely not on Harden's level but he's the same type of player and he can fill a similar role. I think people may be overreacting to Harden being gone. Don't get me wrong, he was a key player for that team, but Kevin Martin is good enough that he can offer 16-18 ppg this season, similar to Harden's production last year, albeit with less efficiency. Not only that, but the Thunder still do have a Kevin Durant and a Russell Westbrook in their primes. Making this trade gives the Thunder cap room flexbility in not giving Harden the big money deal on top of Westbrook/Durant/Ibaka's contracts, and also continues to build for the future in getting Jeremy Lamb (picked just outside of the top 10 picks in this year's draft) and two solid mid-first round picks. That's the way the Thunder likes to do things, and it worked in their favor with the Jeff Green deal. Hopefully, it'll work out the same way with this trade too lol.
As for Houston, they made an f'in great trade. Yes, Jeremy Lamb was a really good prospect. Yes, they needed Kevin Martin's perimeter scoring this year. However, they traded those two for one player who is both a really good prospect and a really good perimeter scorer, one who offers more of both of those things than the two players they shipped out. It hurts that they already gave out big money to Jeremy Lin and Omer Asik and will now have to give big money to James Harden, but Houston definitely needed that star player and it's too late to do anything about Lin/Asik now. Financially, it makes a bad situation worse, but I see it as making the best of a bad situation. Great trade for them, although they will still struggle to make the playoffs this year because of overall inexperience and also the strength of the Western Conference.
As for the winner of this trade, I'm going to take a gamble and say OKC. They lost a surefire stud so that they can bet on Kevin Martin for the short-term and Jeremy Lamb for the long-term. It hurts, but the financial ramifications of the trade are great for OKC and it's the kind of deal that can let them sign that perfect free agent after Kevin Martin's $10 million contract expires after this year. The youth is an added side bonus. And of course, we still know OKC will be up there as one of the best in the West even after the trade. While Houston got a young star, it prevents them from continuing to build, and short-term, they will struggle. They have a lot of prospects they could develop nicely for them, but that's all they can rely on now. There's too much uncertainty remaining with the Rockets to say they won, although they did make a good deal considering what they had to work with.