I think your going to find that the opinions and the results of use are a varied as the components that are connected. All that you can do is to buy a card, and then try your components.
-If you have primarily USB 1.1 components, and are in need of more ports, use a powered hub instead of the pci card.
-For USB 2.0 devices, self powered units such as printers, scanners, readers, etc., leaving the usb connected during shut down or sleep is possible, depending on the device.
-My HP printer connected to my pci card causes no problems. You just have to experiment.
-External hard drives, though self powered, will always pose a problem, and should not be connected to the PCI card. Use firewire, instead. This also allows the ext. drive to be a start up drive.
-USB powered devices such as iPod, card readers, keyboards, must be unplugged prior to shutdown or sleep.
Why the problem? The reason for all of this is, that when instructed to sleep or shutdown, the motherboard tells all peripherals to power down, and then powers down things attached to the motherboard, such as the pci cards (and all attached).The motherboard still waits for a signal saying "I'm asleep" from the pci devices, BUT, the pci is powered down, and no signal comes, so the system won't deep sleep and wont wake.
Bottom line, if you want to use USB 2.0 on an older Mac, you will have to experiment and ever so slightly modify YOUR behavior, so that you don't modify your Macs behavior.
By the way, did you look at this:
http://www.xlr8yourmac.com/firewire/fwusb_card_deepsleep.html
A lot of people have a lot of varied issues and success. You to can be successful, but to what degree depends on you and what you plug in.
G4 AGP(450)Sawtooth Mac OS X (10.4.8) 2ghzPPC,1.62gbSDRAM, ATI9000, DVR-110D,(IntHD)120&160,LaCie160,23"Cinema Display