When upgrading OS X , the cost of the OS is the least of your expenses. There's often software to be replaced , an immense amount of time lost to debugging , and the risk that hardware will need to be replaced.
I figured my 3 yo multi-function Brother MFC-7820N was at particular risk. Of course the paper feed has been having trouble, so maybe it's near the end of its lifespan anyway (versus the LaserWriter 360 that preceded it and would have worked for another 20 years if I'd been able to find toner).
In fact the printing worked out of the box with native drivers. There are supposedly native drivers for OS X fax send as well, but I haven't tested that.
After the 10.5.5 update Image Capture even found the device and could drive a scan, but that might have been a residual effect of the 10.4 install. I don't know if that would always work.
The 10.5 update deactivated the Brother "control center" that enables the push-button scan to server feature . Happily, there's a great article on how to restore this functionality in 10.5.5: Inspired by I-57- Brother MFC-7820N and Mac Leopard 10.5 .
Briefly, you download the latest drives from Brother's MFC download site, install, then use "DeviceSelector" to point to your network device. See the Inspired article for details.
I also did a firmware update using Brother's Java updater. I had to reset the printer but power cycling it, but then the update worked well. Remove the phone line (!!) to prevent dangerous interruptions, don't pull the power line, don't mess with your Mac during the update. I'll see if this helps with some printer quirks (such as having to power cycle after scanning to restore printing!).
Credit to Brother for continuing to provide drivers for the scanning button. I don't care for how the software works (basically running an obscure server on the desktop, doubtless a big security hole), but it is a very nice feature [2, again].
 Actually most of my old software worked fine with 10.5.5, even s/w that didn't work with 10.5.1.
 Why doesn't anyone sell a scanner with embedded Linux that would scan a PDF to a thumb drive?