Hello, Shjak. Excellent question. In my opinion, the extended version is well worth it. Here are my biggest reasons why:
1. The 3D model support, while not (yet) as full-featured as I'd like it to be, is an invaluable time saver. Without it, you need to keep a separate 3D preview program open while you're working on clothing & skin textures if you want to be able to see your work on the avatar model. Then, to see your updates, you need to do quite a bit of shuffling back and forth between that previewer and Photoshop, hitting the various Save and Update commands in each program for each texture. But in Photoshop Extended, all you need to do is hit the Save command once, and the display on the model updates instantly. It's great.
If you're doing the 2-program-shuffle, it takes several seconds to update your preview each time, vs. about a tenth of a second simply to press ctrl-S in Photoshop Extended. That's not a huge difference if you're just doing it once or twice every hour or so, obviously. But if you're like me, and you work by constantly making subtle adjustments to each texture, which you need to see right away, the difference in time spent is immense. If you make a dozen or so small adjustments within a few minutes, up to half your time could be spent just dancing around the various Save and Update commands in each program. Do that for an hour, and you've just wasted anywhere from $20-$100 at professional billing rates. Within a day, you'll have more than paid for the difference in price for PS Extended. The ability to have the model right in Photoshop couldn't be more important.
2. I find the extra measurement and analysis tools to be highly useful, especially when combined with Extended's vanishing-point enhancements. The ability to measure in perspective is fantastic, again a tremendous time saver over doing the old fashioned way. I use it constantly, as much of my work often involves replicating RL buildings and other objects in 3D from photographs. Again, the time this saves on any one project more than pays for the price difference.
3. I LOVE SMART FILTERS!!!! I can't say that loudly or emphatically enough. The ability to work non-destructively with filters is so incredibly important. Frankly, it's long overdue. The power to change the effects of filters at any time throughout the work process is amazing. It's something I've been wishing for for years, and now that it's here, I really don't know how I ever lived without it before. Instead of making different versions of a texture from scratch, you can just have one source PSD with all the needed filters in it, and then just change the parameters, or even the order of filter chain, as needed for each varient. Once again, this more than pays for the price difference all by itself, just by the time savings on any one project.
4. The enhancements to cloning and healing are wonderful. The preview overlay is another one of those "no idea how I ever lived without it" deals, a huge time-saver, which alone can more than pay for the price difference. All you have to do is try it once, and you'll never want to go back. And the ability to transform the cloned area on the fly as you work is huge.
There are of course many other features of Extended that make it worthwhile as well. Those four are just the ones I use all the time. Download the 30-day trial, use these things for a month, and you'll never want to use the unextended version ever again.
You might also want to read this PDF, which offers an excellent overview of Extended's unique features:
http://www.adobe.com/aboutadobe/pressroom/pressmaterials/pdfs/cs3_photoshop_extended_whatsnew.pdfAnd to see a few of them (very briefly) in action, these quick videos give you a small taste:
http://www.adobe.com/products/photoshop/photoshopextended/features/@Atom: I hear you on the print advantages, but I must say you're setting your sights a bit short when you say that print separation is only significant lead Photoshop has over its competitors, and that that's the only reason it costs what it does. As one professional to another, let me state that almost none of my work involves print, that I'm familiar with the ins and outs of a great many graphics applications, and that were Photoshop to disappear tomorrow, I would be at a quite a handicap compared to where I am now. There are so many uniquely powerful features of Photoshop I depend upon. To work with any other raster editor would be a significant slow-down.
As for your suggestion that upgrading to CS3E isn't worth it for Web artists, I would strongly disagree, but I'm wondering why that's even relevant. The OP wasn't talking about Web graphics. He/she was talking specifically about texturing for SL, a task for which CS3E offers many enhancements over CS2.
@ Gusher: Look, we all know GIMP is free, and that it's a good program (if you don't mind its bizarre interface), and that's all great. There can be no taking that away, and GIMP does absolutely deserve every praise for it. But none of that means Photoshop is a waste of money, so you'd do well not to suggest that it is. It is a fact that GIMP does not have any of the features that PS Extended has, so you're not even comparing apples to apples. I can promise you that in two to four years, if and when GIMP catches up to where Photoshop is now (historically, that's been about how far behind PS GIMP has always been), you won't know how you ever lived without these features either. But in the here and now, there is no comparison. You do get what you pay for with Photoshop, and then some.
It can only be a waste if you don't use it. For anyone who does use it, it's worth every penny. Believe me; if there were a free alternative that were as good as Photoshop, I'd be first in line to grab it. I'm certainly not rich enough to shell out $600-1000 lightly. But the fact is there isn't such a program, so I spend what I must. It's more than worth it (again, if you use it). No offense, but to profess otherwise as you did is to suggest strongly that you're coming from a position of ignorance on the subject. It looks far more like fanboyism than educated comparison.
As for Unwrap3D, I never heard of it before your mention of it here. I'm looking at the website now, though. From what I can see, it looks to be a decently featured, very inexpensive, UV editor. At first glance, I'd agree with you that it's probably a worthwhile purchase for any Windows user who doesn't already have a full-featured 3D modeling program with built-in UV editing. I'd have to try it out before I could say for certain how well it works, though, obviously. If I find a little time to play with the demo, I'll post a review of it here. I encourage anyone else to do the same.
But as for your "I'd rather buy" statement, you're sounding like you feel it has to be an either/or choice. I seriously doubt that the fifty bucks for Unwrap3D would make or break anyone's Photoshop purchasing decision.