I got it because when my son went to college, he got a MP750, and I really like it. Printer, Copier, Scanner... the MP780 is also a fax machine. This was two years ago.
If you do not need to have the machine on ethernet to share with others, take a look at the Canon Pixma MP530 or MP830.
http://www.usa.canon.com/consumer/controller?act=ProductCatIndexAct&fcategoryid=178
I can't talk too much to the Windows issues you mentioned, though I have used my work-issued Dell and HP/Compaq laptops to print to the MP780... I never tried scanning with them. I regularly use my G5 iMac flatpanel and G4 QuickSilver tower (and when my son visits, his PowerBook G4) on the MP780 (and he regularly uses his G4 PowerBook and iMac CoreDuo with his MP750 at school).
Rather than use print sharing, I just put a manual USB switch on the printer... because of the scanning feature (I think), all sorts of stuff gets strange when it is on an ethernet network.
I need to get a new printer/scanner for my PowerMac G5 and was looking for some recommendations. I currently have a fairly new HP PSC 2400, but I am taking that and dumping it in the river. The software never worked right and always crashed. I just updated it, and now I can't printer, has no options for 4x6 photos and trashed all sorts of stuff on my system. My father tried running the same printer under Windows and he can't do anything (he gets "scripting errors" in Internet Explorer when he tried to scan!)
I am looking for what people would recommend. I need something that can print and scan. I want to print normal paper (both color and BW) as well as high quality photos on glossy paper. Oh, and obviously USB2 connection.
I want something that works very well with Mac OSX and follows the Mac standards (as HP doesn't). I DO NOT WANT ANY RECOMMENDATION ON HP PRODUCTS. I WILL NEVER BUY ANYTHING FROM HP EVER AGAIN!!!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic