Im a recent convert to manual (started this year) and I LOVE it. first of all, without manual most games were too easy. Not that I would win every game but I would finish in the top 4 of the EPL with say Wigan. I wanted a little more challenge than that so I switched things to manual.
With manual you control the absolute power and the absolute direction of passes/shot/crosses. Where you point it and how long you hold it determines exactly where it goes. It is much harder to ping poing around the pitch and score using manual. I do like that you never pass to the wrong guy as you do with assisted and if there is a mistake its MY mistake not the assistance of the computer. I would recommend making the switch if you want a more realistic/challenging experience. I even use manual when I'm playing online against guys who aren't. I just like the control that I have, win or lose.
Sidenote- I keep my shooting setting at SEMI. I'm not quite good enough around the box to score with any sort of consistancy on manual shooting. Hope this helps.