iOS 8 has a ton of bugs. I have been using it for most of the summer through my Developer Account. There were actually very few betas for this release...makes me wonder if they wanted to just get it out in time for the new phones.
Either way, Wifi is slow or doesn't connect, apps crash like crazy, and battery performance is a mess. Location Services keeps running in the background for system services too, which is strange.
iOS 8.0.1 which was released today is a mess too. The over-the-air update through the phone or iPad system preferences caused the devices to drop cellular reception and Touch ID capabilities.
Is there any way of re loading the previous iOS ?
Yes, there's three ways of fixing the issue:
1) I believe Apple is still pushing iOS 8.0.0 through iTunes. So, if you've updated to the newest version through Over The Air, then you can connect to your phone and have it restored to the old version.
2) You can manually download the 8.0.1 IPSW file from another source online. Check www.MacRumors.com
in the iPhone forums for download links.
3) Similar to step 2. Find an iOS 7 IPSW file and restore through iTunes using the Option + Restore method.