In particular people who also do a lot of 3D work will like that it has elements incorporated that allow them to use it to edit and even render some 3D projects. For a photographer, I'm not all that sure that CS5 is necessary. I have found perhaps one inclusion that I'll use, however, I've also discovered some things missing from CS5 that I used to use on a fairly regular basis.
In particular under the "automate" functions there is no longer the ability to create a contact sheet (proof sheets) for photographers, nor is there the ability to automatically create a "picture package" for printing, nor is the web photo gallery function available. Also missing is "export layers to file" and "export paths to illustrator". The "export layers to file" function is under scripting, so it should be simple enough to copy and install the script to make it work in CS5. I suppose what I find annoying is that I have to copy all these things into my new version.
The good news is that you can get these functions back by copying their folders from your older version of Photoshop into the same location in Photoshop CS, so if you copy the web photo gallery folder into the "automate" folder in CS5, you'll be able to use it again. The same goes for the picture package and the contact sheet. I loaded these into CS5 and tried them all - and they work just fine, thank you very much. I can only guess that they've removed them from CS5 because they incorporated them into Lightroom, but not having Lightroom, I don't know for sure.
There are some things in CS5 that I don't like - for instance the new brush selection. I think once I've spent some time using them I'll be a little happier with them, but at first glance into the brushes one might think "where's my brushes?" That's because the new brushes are simpler - round soft or round hard, shaped brushes for painting and design, and not much else.
With the new brush selection you pick soft or hard, and set the size of brush you want using the slider. That's slightly annoying to someone whose used to selecting a 300 pixel soft round brush quickly. The good thing about the brushes is that once you've selected your brush and set it's parameter you can save this brush with a click to your brush file. And of course, you can copy all of your old brushes into the brush folder of CS5.
In CS5 I'm having issues with the brush display when using a brush - rather than seeing the round outline of the brush I'm using, I can only see half of it. That doesn't happen for all brush sizes, but once I move the pixel size of the brush over 50 pixels, that's what happens. This makes it very difficult to work with and this is the one aspect I truly hate about the brushes. CS5 uses some openGL for display, and indicates you might have display issues since it uses more memory, and recommends changes to avoid that - I followed their recommendations and I'm still having display issues (and yes, I'm pretty sure I have enough memory for this).
The other thing I dislike a whole lot is the clone stamp - this does some funky stuff now. When you select the portion of image you want to copy FROM and hover over the spot you want to copy TO, it shows you what you're cloning (from). It's more than a little weird right now. It also seems to retain the copied sections so the next time you select the clone brush, you'd see in it's "preview" your previous clone selection. It's going to take some getting used to.
I also don't like what happens when you open an image. In Photoshop CS3 when I open an image into CS, the image window can be maximized to fit the workspace area of Photoshop. In CS5, whenever I try to do that, the image window opens full screen, completely covering the tools and menu in Photoshop. I continually find myself opening the image, then grabbing the window edge and dragging it open to space I want it to fill. I have changed the workspace and tried different options, but haven't had any luck in getting it to open the way I want.
Image open full in Photoshop CS3
Image open full in Photoshop CS5
There are a couple of my older plug-ins that don't want to work in CS5 (notably "Genuine Fractals PrintPro") which means another upgrade if I want to use them in CS5. The one thing that's really nice is that CS5 installs to it's own folder in the Adobe program files folder, so I can still open and use Photoshop CS3 (which I've already found myself doing), so for now, until I'm somewhat happier with CS5, I'll be using CS3 when I need to upsize an image for print.
Oh...and the one thing I thought I'd use in CS5? That would be the new "HDR Toning" - it's not bad. I had already purchased something that does this just fine for previous versions of Photoshop, but CS5 has it incorporated into the Image Adjustments menu. It has some pre-sets you can use, but the custom adjustments allow you to use (faux) HDR tone-mapping on a single image and get a "cartoon" type image, or photorealistic image, or fantasy style.
HDR Toning in Photoshop CS5
To be fair to Adobe...I am somewhat resistant to change, and I haven't really spent enough time with CS5 yet to be comfortable with it, but I suppose the whole idea for me is that an update shouldn't require a user to practically have to start over learning how to use the software. It just shouldn't. CS5 has a new look on the interface, which might throw some of the dinosaurs (like me) at first...I didn't like it at all, but much of the stuff is in the same place as the older version.
What I'd recommend for anyone who hasn't upgraded yet to CS5 is not to purchase the upgrade straight-off. Download and use the trial version for as long as it allows, and then decide whether the cost of the upgrade is worth spending for your particular use. For me...so far, I don't think it was. I wish I could get my money back.