Buy a new router.
Wifi is working fine for the majority of people and has for months.
That's not the question! The question is:
- Does the router work with other hardware?
- Does the router work with other OSes (like Windows)?
- Does the same router work with the same kind of hardware, but with running a previous OS X (e.g. Snow Leopard *cough* *cough*)?
- Heck, does the router even work on the exact same machine that is having Wi-Fi issues under Yosemite, but with another OS (e.g. Linux)?
And guess what, back in the days, under 10.9.2 or so, I went through the exact list as above, and each question was answered with a YES! Yes, yes, and yes! Wi-Fi works under all the above scenarios, even on the same ****ing Mac - with Bluetooth enabled! - under Linux! But it wouldn't work under OS X 10.9.2.
(The problem was even more ridiculous: it worked on the 5 GHz band, but not on 2.4 GHz - and don't tell me I should have disabled Bluetooth under OS X: because I did! Changed channels, changed every reasonable knob and bolt there was).
And guess what: yes, I eventually bought a new router! I needed a new, more powerful router, which could operate under 2.4 and 5 GHz at the
same time. And it worked also with - tataaaa - OS X 10.9.2 (later updates supposedly improved Wi-Fi, but I never cared to test again with my previous router, which by the way was an otherwise totally reliable and up-to-date patched FritzBox).
(To be fair: the new TP-Link would switch channels automatically a bit too sensitive, and each time the Wi-Fi connection was interrupted for 5 seconds or so. So I set channel selection to "manual", and be done with it).
But the lesson was: Apple really ****ed up their network stack since Lion! And if people are still having Wi-Fi issues, even under 10.10, I can totally feel with them!
Yes, buying a new router might solve the problem. But Apple should simply FTFWF (Fix Their ****ing Wi-Fi)!
(That's not the end of the story, off course: I could go on that even under 10.9.5 I can still bring down the Wi-Fi to a total halt "reliably", e.g. when backing up my iPhone via iTunes (the backup is stored on a NAS in the end), and at the same time transferring some other large (in the orders of GB) file. Or importing photos into Lightroom, which under certain circumstances kills the entire Finder and I have no choice but to reboot. Etc. etc. - but I won't. Enough bitching...)