IOS8 on my 64GB iPad3 is terrible, sorry to say. Maybe it's just Safari. Page loads take forever. The copy / paste issue hasn't been fixed. Then there are the myriad usability issues like swapping the bookmark and send icons in the menu bar. I haven't had the WiFi issues that others are reporting, though.
All in all, I can't say that IOS8 on my iPad3 has been a positive experience. Now, Safari on my rMBP with Yosemite IS a positive experience, though I still prefer Firefox.
There has been some scuttlebutt that Apple intentionally put features into IOS8 that would cause it to run slower, in the hopes that people would get frustrated with the performance to the point where they'd upgrade to a new iPad. I can't believe Apple is that conspiratorial, though it might be the case that some features are, in fact, more system resource intensive. But if IOS8 ever gets to the point where it is totally unusable on my iPad3, there is no way I'd get a newer one. If Apple wants to obsolete their devices like that, then they'd lose me as a customer. At least for tablets, anyway.