Problems that I have had have been minimal, a few app incompatibilities, but nothing major. To be honest, it's not much different to using Tiger, the reliability seems to be similar, and the bugs are annoyingly similar.
Such as the SMB bug, when connecting to a Windows machine and the connection hangs up for some reason, Finder hangs and there is no way of recovering it, hard reset is the only way. I experienced that one last night, which pissed me off somewhat. :-s
I have an idea how to get round that one though, make sure you have a terminal window open so you can kill Finder or halt the system if it happens. ;-)
Apart from that, that's the only complete system hang that I have had since the iChat backgrounds facade 2 weeks ago.
Memory usage is pretty stable, Safari seems to eat a lot now and again, but then seems to release it back too now and again, but thats web browsers for you in general. Load wise, the system isn't really doing much, and the load is minimal.
Few app incompatibilities have been noticed, such as X-Lite (softphone) not working at all, but apparently a new update will fix that. ClamXav's scheduling also doesn't work properly, although, I don't really care. I'm quite capable of updating virus defs manually.
Everything else appears to be working fine.
Time Machine is brilliant, I have used it several times to go back to prev versions of files that I have been hacking about with, and I also used it to do a full system restore, which worked flawlessly.
It seems though, many other users have not been as fortunate as me, if you go on the Apple support forums, it's full of people with various problems. My honest belief is that these people are creating the problems themselves, by doing upgrades for a start. Who in their right mind upgrades an operating system? God, no...not a good idea. Always start fairly clean at least, I think archive and restore is the way to go personally.
Also, people don't think to remove all the silly hacks they implement. If you want hacks, at least expect some system instability. It seems that many users prefer to mess about with the internals of their systems, which is up to them, but, what do you expect when you do an upgrade?
Also, macosxhints.com, I don't think they have actually published a decent hint for Leopard so far, most of them have been complete crap, or ugly hacks that are most likely going to be zapped by 10.5.1.
So, if you want my advice, don't bother.
10.5.1, by the way, has been seeded to developers, so i'm sure it wont be a million years till it's out. It may well come out at the same time as 10.4.11 and Safari 3 Final. In fact, 10.4.11 will probably include Safari 3 final on Tiger.
I'll post some more at some point when I can be arsed, but those are my thoughts for the moment.
Bye for now.