I just recently started using CS5 after having been a CS4 man since it was in beta. Most of it is the same, so this won't be a long post, but there are a few things which I thought some of you might find interesting. First off a pet peeve from either version of PS. What the hell is the point of the 'pixel grid'? At the point at which it shows up, you can plainly see the square block of the individual pixels. The extra grid lines only make it more obscured and harder to see. I don't understand why the default is to have it turned on. Ok, end rant.
Let me say that I didn't switch from CS4 for any particular feature. Being a Windows user most of the time, I've had 64bit Photoshop for years now. In fact the main reason I switched wasn't a feature of CS5 at all, but rather an annoying feature of Lightroom 3. You see if you're using LR3 with CS4, when you open an image in PS by using the "Edit in Photoshop" command, Lightroom renders out a PSD file (or TIFF I guess, but I use PSD) to the disk and then opens it up in Photoshop. This is all well and good except I often open a number of images to grab different pieces to use in a composite so I'm left with a bunch of big 16bit PSD files I've got to go find and delete when I'm done. This has to do with CS4 not supporting the latest version of Camera Raw. I'd like to point out that I think this is pretty crappy of Adobe. I can't believe there is a good technical reason they couldn't make the latest Camera Raw plugin work in the last version of PS, and having to spend hundreds of dollars on an upgrade just for that is annoying. That said, CS5 fixes this issue for me and stuff opened in LR now just shows up as an unsaved new file in PS.
Now that I'm hear though, there are some new features. The main one people point to is this content aware fill. Overall I haven't had the best luck with it. My results have not been nearly as super as the demos, and besides, most of the time I want a bit more control over what goes in the newly deleted space. For example, using it with the Spot Healing Brush, I found that the old Proximity Match setting works better at least half of the time. There are certain situations where it's like magic, and I'm not one to look a gift horse in the mouth, but I certainly wouldn't say it's worth the upgrade.
I often use the Lens Correction plugin to add vignetting, and it's no longer in the Distort menu but rather just near the top of the Filters menu along with Liquify and the rest. It seems like it's got the same auto controls that now exist in LR3, so not too much new there. Though I'd like to point out that if you fixed the distortion in LR and then open the image in PS and run Lens Correction, be careful because you could double correct and screw things up. It's not smart enough to know that you've fixed the problems already.
The only other thing I've noticed is that sometimes my preferences are spontaneously reset, including that damn Pixel Grid and it's new devil child Scubby Zoom, which is just about the most annoying feature ever. You use the zoom tool (magnifying glass) and click and drag to zoom in and out. For me it's so fast and uncontrollable as to be useless. I always end up zoomed in so far I've got 3 pixels on my 30" screen. Very disconcerting.
Overall I haven't noticed any great performance or UI enhancements. Let's be honest, Photoshop is pretty mature at this point. It's like getting a new version of Word. Most of us use about 10% of the app as it is. We really don't need more. Your milage may vary.