I'll throw my 2 cents in. Brand new MacBook Pro (September 2014), fully loaded with all options (i7 quad core, 16gb of RAM, 1TB ssd drive, etc.). Upgraded to Yosemite last night. Install worked without any issues. Setup after install was fine. Apps seem to work fine.
Safari and Chrome both were terribly slow to resolve to any pages (like a DNS issue). Also, Pandora had a hard time resolving (through PandaBar). Pandora sent me an email saying that they kicked me off b/c they saw me connected through 2 different devices (which I was not, just an issue with the MacBook not connecting/resolving to their service). Google.com would come up but searches would take 3 minutes or more to come up once there.
Tried to print to my network printer. Printer is "online". Print job sat in the queue for about 2 minutes before making it's way to the printer. I can pull up the printer through the network without any issues. Deleted the printer and reinstalled without any problems, still 2+ minutes for a print job to leave the queue to go to the printer.
I have the laptop connected to an Apple display via thunderbolt. I plugged an ethernet cable into the display, passing the hardwired ethernet via the thunderbolt to the laptop. Internet works without any delays.
Printing, however, is still seriously delayed. Queue just says "Ready to Print" with the document doing nothing, until it is about 3 minutes later, then it says Printing, Looking for Printer. And within a few seconds the print job is done.
I tried a DNS flush (sudo discoveryutil udnsflushcaches), but that did not seem to fix any issues. I will note that when I searched online for DNS issues, people mentioned mdnsresponder and dnsdiscoveryd services, but I do not see either of those running on my machine in Activity Monitor (with all processes displayed).