For what it's worth, I have an iPad 2 and an iPad Air for work, and I've left the older iPad alone due to a lot of complaints from other users. I'm not too surprised. It does seem like the iDevices are only good for one OS upgrade, then hit that window of being bogged down to total shit.
The iPad Air has been fine on 8.0.2, and I'll put 8.1 on it later today to see how that fairs. The speed is unchanged, the battery life improved slightly and I have far fewer crashes with apps than I did on 7.1, so for Air users, I'd say go ahead and update to at least 8.0.2.
My work iPhone 5S is on T-Mobile, and the best feature of iOS8 is the WiFi calling feature, but that's only available for T-Mobile at this time. Shit, my battery life is through the roof now. Whenever I'm home I just stick it in airplane mode with WiFi turned on and I can make calls all week, and sit in standby fine, and usually still have power left over by the weekend. It's freaking nuts how much better WiFi is than cellular data for standby time. I really hope more carriers enable that in the future. I was charging that damn work phone every other day before. To get a week+ out of it now is insane. Totally made 8 worth it.
5 and older iPhones and iPad 2 and older devices, I've heard of nothing but headaches with 8.x. Funny thing is... Stupid OS really doesn't look any different, or seem to do anything new or flashy. It has a battery summary now to show you what apps are taking the most power, and... They hid all the damn photos, but other than that I'd be hard pressed to tell an 8.x device from a 7.x device, so not sure how the hell these things were slowed down so much.
And... before anyone asks... I manage all of my company's mobile device releases on iTunes, Google Play, Amazon, Samsung and the Windows Store... So yes, I have a shit ton of devices
No, I wouldn't have bought all of these myself.