In terms of Portal simply being a fantastic stand-alone experience that needs no sequel to complete it? Kind of. It absolutely is, except for the whole "still alive" thing. It was obvious, even before the updated ending, that GlaDOS was not dead, and more story potential was there. Not to mention the potential for further ties with the Half-Life story.
In strict gameplay terms: absolutely. Portal was an incredible game with some incredible ideas, but that's not to say there isn't more that can be done with the concepts. There is still a lot more room to explore, and I'd rather the makers of Portal do it than some random developer hoping to get a quick buck off a cheap rip-off of the first game.
So overall, yes. And regardless, IT WILL BE AWESOME.