I have the M8 coming from Galaxy S3 since August 2012. The M8 is lightning quick with no lag which is to be expected I would imagine of most of the quad core monsters coming out within the last several months are no different in that regard. My S3 was maddening when scrolling through photos for example, it would lag and lag and the camera would constantly crash. What I really enjoy about the M8 is how truly smooth it is. There is no lag whatsoever, ever. I have always had Samsung smartphones, the S, S2 and S3. I played around with my co-workers S5 the other night for five minutes and didn't see anything ground breaking compared to the S3 other than it was much snappier and faster of course. I figured why not try something new and for the most part I am pleased with the M8, SENSE 6 has been easy to get used to for the most part. I do miss some of the features of my S3, particularly with regard to the phone app such as the way the contacts, favorites and so forth are laid out.
Either way I am sure I will be lusting after something else in 12 months or less, so in the meantime I am content with the amazing upgrade from the S3 and plan on enjoying it for the time being.