The "precision" of a controller will not be nearly as good as what can be achieved with a mouse. That little thumb stick will not allow for the precise movements that a mouse can . A mouse just has better innate accuracy. A mouse maps input directly to the movement on the screen.
A mouse and keyboard are the better tools for playing FPS. Any time precise aim is needed, a mouse is going to be a better choice. It is the same as a wheel and pedals being better for racing games, a controller being a better option for playing a game like FIFA, or a flight stick being better for flying games. Using an inferior tool for the job lessens the potential of the user.
I was a long time console player. I saw little need to play on PC when the console experience was so great. I had done some PC gaming, primarily MMORPGs and RTSs, but only because a mouse and keyboard are the better tools for that type of game. All of the extra keys are needed to quickly utilize all of the abilities available in those type of games. Controllers just do not have enough buttons available for key bindings.
I made the switch to PC gaming during the Battlefield 3 Beta. Making the switch to keyboard and mouse, movement with a keyboard gave me all kinds of trouble. I constantly mixed up crouch and prone and all of the other key bindings. There were just too many other keys available to do some many different things. My left hand and brain were just too easily confused. While I mostly no longer have those problems, I now sometimes have issues with hitting the correct number keys to switch between weapons or hitting keys away from the normal keys that the fingers rest on when gaming.
However, that is more of a function of switching from a laptop keyboard to a mechanical keyboard. The layout and feel of the keys is just different enough to cause me some occasional problems. I am adjusting and mostly have it down, but the issue still pops up every so often.
Quick detour, but a mechanical keyboard is so much more awesome than a membrane keyboard. Buying a mechanical keyboard has ruined all non-mechanical keyboards. The tactile response offered by a mechanical keyboard is amazing. Membrane keyboards feel mushy and the cheapness of the membrane design is very apparent.
The switch to PC gaming was easier with a mouse. I have been using computers for long enough that aiming with a mouse was fairly natural. At first, the reflexes were completely not there and I was getting stomped because many of the players I was going against have been playing PC FPS for years, but those reflexes came with playing more. I am still by no means a great player, but I am improving and can somewhat hold my own. Going up against very good to great players can still be very frustrating. Those guys are just so precise with their aim and so fast with their reflexes.
Lastly, it did not take as much time as I thought it would to adjust to using a mouse and keyboard. It required a period of adjustment where playing with the different tools was at times incredibly frustrating. However, the payoff for playing with the correct tools for the job was/is well worth the effort.
The RFB was my favorite weapon, kept it on iron sights. I was quite effective with it. Didn't like the Mk 11 though, it didn't feel right.
