I've got all five seasons on Blu-Ray and the picture improvement is impressive. The only problematic thing about the picture quality are the effects shots.
Must of them were done in a lower resolution when the show was produced and it shows sometimes in the first two seasons. It's not so noticeable after that but still obvious sometimes, especially when it comes to establishing shots of Atlantis. Those are soft during all 5 seasons.
But that really shouldn't keep you from getting the Blu-Rays. The overall quality is leaps and bounds better then that off the DVDs. The DTS HD Master Audio Tracks are also quiet good for a TV-Release.