Here,this is the tutorial that i followed, you don't need an unlocked phone, there's is a way(is in the tutorial) to register for developing for free:
nokianesia blog: Try GDR 3 / Update 3 Developer Preview for Nokia Lumia Windows Phone 8
As for cons and pros, for my case, i consider myself lucky, so far, i have zero bugs, i fact, i have only gained new features and the phone is improved, before the gdr3, my phone would restart it's self at least once in a week, but after i updated, no more restarting, also, my phone used to heat a lot when using 3g and browsing,that doesn't happend anymore. There's a post here in wpcentral with the developer preview bugs, you should check it, oh, and now there's a status bar during the update, so you can now know how long it will take :)