I just had to do it—install Leopard the day it came out, that is. And now I'm typing this with both the caps and num lock LEDs on my keyboard turned on but the functions themselves turned off. I don't know who is in charge of new bugs in Mac OS X, but it's surely someone with a strange sense of humor.
Fortunately, Leopard brings more than just new bugs. Apart from everything you can read about in John Siracusa's epic review, there are some less visible features in the networking department. First of all, there's the new Network preference pane, which is a joy to use. It's now much easier to quickly see if you have a working IP address. If you're into Windows networking, you may be interested to know that the "Advanced…" button hides—among other things—a new WINS section where you can set up a NetBIOS name (by default, the name you set up for your machine under "Sharing"), a workgroup, and one or more WINS servers. The advanced settings now also include both the IPv4 and IPv6 configuration settings, although for some interfaces (such as Ethernet), the IPv4 settings are duplicated in the non-advanced part. It looks like the 802.1x capabilities are greatly expanded, which is probably a good thing for people who need to connect to corporate WiFi networks and the like.
Somewhat hidden under the list of interfaces, there is a little pull down menu that has something we haven't seen or heard about before: an option to "manage virtual interfaces." When you select this, you can set up VLANs and "link aggregates." Almost all switches except for the cheap consumer stuff make it possible to configure groups of ports into virtual LANs. It is then also possible to make one port a member of several of these VLANs. Leopard lets your Mac connect to multiple VLANs through a single ethernet interface connected to such a port. Or you can go the other way: connect to the same network using multiple ethernet interfaces, if you have them. That's what link aggregation does, presumably in an IEEE 802.3ad compatible way. This gives you extra capacity and it's safer, because now two cables must be unplugged before you're disconnected from the network.
One of the 300 new Leopard features is automatic TCP buffer sizing. Originally, TCP was designed so that it could send out a maximum of 64K data before it had to wait for the other end acknowledge receiving some of it. On a LAN where it takes less than a millisecond to receive that acknowledgement, this works very well. But I happen to have a server located a thousand miles from where I live. It takes 40 milliseconds for a packet to travel from my Mac to the server, and then for the return packet to travel back. (Both are connected using 100Mbps links.) Under Tiger, the buffers that limit the size of the TCP window are 32K, limiting to 32K every 40 ms = 800 kilobytes per second. In practice, I got 700KB/s. With Leopard, this increased to well over 3MB/s.
Automatic TCP buffer sizing should help make some downloads faster using Leopard, as long as the server is also set up to use larger buffers. Vista also uses bigger TCP buffers, but Microsoft ran into problems with certain firewalls. It's unclear whether the firewall vendors involved have all solved these issues.
And last but not least: Apple's Mail can work over IPv6 again! I first started to get serious about IPv6 during the reign of Panther. After I installed IPv6 mail daemons on the server I mentioned earlier, I was able to both send and receive mail over IPv6. But then Tiger came out, and sending mail over IPv6 no longer worked. The better part of a year later, this was fixed. But it broke again when I got an Intel Mac this summer. And now it's fixed again. Let's hope it stays that way.
In addition to massive speed increases and the above mentioned bug fix, Mail also got a small-but-useful new feature for IMAP users: it now supports IDLE. That means that the server can tell Mail there's new mail, so it's no longer necessary to set a very low new mail check interval.