I understand the value of using a tablet like a Wacom Intuos for Graphic Design. But with Photography Editing how useful is the use of a tablet?
You won't get much out of the pressure sensitivity, granted, except when doing things like painting masks (where being able to vary the opacity with pressure while you use the bracket keys for brush size as you go makes things a lot easier). But the absolute-coordinate behaviour is very, very easy to get used to and spoiled by -- you always know exactly where your cursor is, and you develop muscle memory for the location of tools and palettes very quickly. Once you've used a tablet for a couple of weeks, going back to pushing a bar of soap around a desktop with no physical reference points feels almost like something's been amputated.
That said, the Intuos is probably overkill unless you are doing a lot of compositing or want access to keyboard shortcuts directly on the tablet. (You can more or less push the keyboard out of the way altogether once you've got the buttons and spinner programmed on the new Intuos models.) The Bamboo pen tablets -- or even an old Graphire, if you can find one used in working order -- are probably enough. I got by with a 3.5" x 5" Graphire for a lot of years (in a lot of ways it was a better tablet than the larger, original serial-port Intuous I had before that). It's the absolute coordinates -- where the pen is on the tablet is exactly where the cursor is on the screen -- that's the "killer app" part of the deal. I don't know if it's possible to relate just how much that means in words; you really need to try it for an extended period. The rest of the features on the Intuos (compared to the Bamboo) are merely conveniences (although there's nothing wrong with things being convenient).
I agree with Stan, but you may also want to consider the idea of a tablet PC. I have one, an Asus, with the touch screen and pressure-sensative stylus, that runs full Windows 7 with a dual-core CPU. This lets me have a full Photoshop install and the ability to use the stylus directly on the image and I find this far more intuitive than the regular tablet (I also have the Bamboo Fun on my desktop PC) for editing photos. As an added bonus, it has a built in SD reader, so it's really easy to transfer images...
I have a tablet, but generally only use it for very specific dodging/burning. Even then, I use it in combination with my trackball. For changing curves and changing the sliders in Adobe Camera Raw, it's my trackball all the way.