I played 1.6 last night, actually. It's a lot different having to aim without CS:S's Magic Bullets That Curve Around Walls to help. Took me a few minutes to get accustomed to it, but every kill I got was much more gratifying.
I just prefer CSS since that annoying Drawback when shot at grates on my nerves. It's bad enough slowing down, but I wouldn't jump a foot backwards aswell.
I play them both, and although valve says the gameplay is identical. I have to say in my experieces you can spray more in CSS, and the galil and famas are far better, while the mp5 is far worse then in cs 1.6. But thats just probably me.