Last week, I watched the Gaikai demo featured on Massively and Slashdot. I came away quite impressed: it looked realistic, didn't make over the top claims and the speed of loading and playing MMO games like Eve Online and WoW looked decent as well. I immediately requested a beta signup, as did many others I suspect!
There is however one issue with Gaikai, that that is immersiveness. Some MMO games or virtual world environments are intended to be immersive: the intention is to immerse all your senses in the game or world you're visiting, creating an intense experience. That's why Twinity, Second Life and Eve Online for instance are best played in full screen mode: nothing of the host computer or operating system is visible on screen, to distract from the environment. From the demo, it isn't clear whether Gaikai can deliver that; I'm actually pretty sure they don't. This is not an issue for most games I suspect, but it should have a negative impact on the immersiveness of virtual worlds, as you can see in the snapshot below.
Still, if the price is right, I would sign up for the service. The ability to access my favorite virtual worlds and games from every PC that has Flash installed, is certainly appealing. Perhaps not as a primary means for real gameplay, but to do a bit of trade, to finalise a deal, to check in on in game communications, to add a skill to my Eve Online skillqueue, Gaikai may still be very useful.