Used the 5D for the past 4 months, here are my thoughts:
Pros:
1. Noise control is excellent. ISO 1600 is very usable.
2. Superb details.
3. Highlights doesn't clip as badly as 1D mk2, slightly more latitude.
4. White balance is really good, except that it tends to be slightly cooler than the 1 series (assuming both 1D2 and 5D are set at AWB, 5D images tends to look a tad cooler).
Cons:
1. USB 2.0 can't beat Firewire in terms of stability. Sometimes had to restart software on my powerbook G4 as well as cam in order for it to work again, else cam just display 'BUSY'. For those who doesn't shoot tenthered, you can ignore this one, hehe. Seldom had that issue with the 1D mk2 and 1Ds.
2. Jpeg file not that great compared to jpeg straight off from the 1 series. Noticeable difference between jpeg straight off from cam and jpeg rendered from RAW. Bottom line is, shoot RAW for the 5D, you won't go wrong.
One thing to note though, to harness the full capabilities of the 5D, you need good optics. Distortion that you've never seen b4 in your lenses will surface... For studio shoot, the 135 f2L stays on the cam most of the time