A few things:
1. Do you shoot at high ISOs? More on this later.
2. Is carrying a heavy camera around too much?
3. Do you need the super AF performance of the 1 series digital bodies?
4. Do you need the weatherproofing/sealing of the 1 series digital bodies?
5. Do you need more than 4mp (more on this later).
6. Do you crop a lot? (more on this later).
The lenses will be fine, they'll work in other bodies. If it's an EF lens it'll be fine. Not unless Canon changes the lens mount 
Price? Anywhere from US $1200 to $1500 I'd say, with the higher priced items having less shutter actuations.
Now - onto noise. Using something like Neat Image or Noise Ninja will fix most of the noise issues. Even then, the noise from a 1D is equivalent to the grain from a traditional 1 series film camera if you ask me. It's just that the 1D mark II/n are really GOOD when it comes to noise, far better than the film counterparts.
Even though the camera is only 4mp, it's a quality 4mp, and the pixel size is nice. The 1D has a better pixel size than any other Canon camera for avoiding diffraction issues if you ask me. The 4mp CCD compares well to a 6mp unit.
if you crop a lot, then make sure you get the main part of the image as large as possible in the frame. Fill the frame! The 1D does have some issues with cropping I've found.
The only issue is long exposures, you'll see the amplifier noise from the CCD sensor and it's unfixable. If that bothers you, get another camera, or go for much shorter exposures.
The image quality is simply outstanding. It's definitely worth the cost if you ask me. You'll be prying the 1D from my cold, dead hands and nothing less.
Cheers,
Dave