I've been using MacOS 10.5 on my G5 PowerMac for the last week now, and so far, I'm not terribly impressed. You can likely find tons of fanboys slathering saliva all over the nifty features, and you can find tons more haters piling poo upon their install DVDs, so I'll just go over the stuff that *I* noticed and that I've not seen discussed in much detail. (Not that I've really looked.)
My G5 has a gigabit interface - two of them, in fact, although I've only used one. It's connected to a gigabit switch and has been Just Fine, Thank You Very Much since I got the machine in December 2005. I upgraded to Leopard, and poof - all of a sudden it's 100mbit. I don't think I need to say how annoying that is, particularly since I'm regularly shipping ISOs around. The other issue I noticed is when the machine wakes from sleep, it takes a full minute sometimes for it to reacquire an IP from DHCP - this part is so annoying that I finally just set my IP statically.
The first time I powered the machine off after installing Leopard (Wednesday or so), it did its shutdown thing, then the sleepy light came on and *all* the fans started running full blast for a full minute - until I got annoyed and held in the power button. That behaviour hasn't repeated, my machine is its usual quietish self, but I guess I'll see if it repeats this weekend.
X11 is really annoying now, from the \$DISPLAY thing to no fullscreen. It broke Gimp.app seriously, and the MacPorts gimp port is busted. I get to shuffle over to my Linux machine when I need gimpy.
I got a 250GB external FW400 drive for the express purpose of trying out Time Machine. I plugged it in, and my system crashed immediately. I guess it didn't like the partition table on the drive (likely had been running an NTFS fs), but experiencing some data loss - fortunately not catastrophic, and I have other backups anyway - when trying to set up a feature that's supposed to reduce this sort of thing is sort of... crap.
Boot Camp. Not strictly a Leopard issue, but since you are apparently No Longer Allowed To Use It since Leo's been released, I'll blame it anyway. I understand *why*, but that's a *very* chintzy move, Apple. I know they're a company that needs to make money, but there's no real reason to do this beyond "we don't give a crap about our existing customers." I have an iMac at home, purchased mid-September, which will be running 10.4 for some time - if Boot Camp has been disabled on it, I certainly won't be forking out \$180 to upgrade, I'll just suffer with VMWare Fusion. Apple didn't lose a sale cos they'd never made it, but they won't be making one and now I'm angry to boot.
The positives from the 'upgrade' so far are few and far between, mostly because I was honestly satisfied with 10.4. The biggest one? I can now - finally - rotate my displays, which again just makes me that much more angry with Apple, since there's no reason they couldn't have distributed an updated driver for my 10.4 system. Thankfully, upgrading work machines only costs \$80 and it's not out of my own pocket.
All in all, if you're not running 10.5 already, I really wouldn't bother. Spaces? Buy CodeTek Virtual Desktop, or any one of the other applications that do what Spaces does except better. Time Machine? Meet Carbon Copy Cloner. Stacks? Give me a break. 297 other features? I've yet to really see them, I suspect that 1-50 are "1. updated Documents folder icon. 2. updated System Preferences icon. 3. updated Downloads folder icon." and so on.