I'm pretty late coming into this discussion, but I would like to throw in my 2c. When I started IMAC flying Basic, I used some relatively inexpensive servos in what I thought was a pretty nice plane. That was probably the biggest mistake I could have made and it was extremely noticeable (but only after I went to good, precision servos the next year). Things I noticed with better servos include:

Centering: servos returning to center precisely. This helps SO MUCH when trying to fly straight lines because you won't always be fighting a plane that is just slightly out of trim (climbing, rolling, falling, etc). The plane should be rock solid in horizontal trim so that your workload is decreased. Ideally on a calm day, you should be able to establish a horizontal line and let go of the transmitter - the plane should fly from one end of the field to the other without you having to touch a thing. Believe me, this makes precision flying SO MUCH easier.

Precision: this is harder to see, but basically you will notice (particularly with 1 elevator servo per stab) very precise control - if both servos track the same, then the looping figures will be much easier to control. Also extremely important in snaps (not a big deal in Basic) because the servos will always move to the same position and therefore the snap will be more predictable and controllable. This will also be extremely noticeable on throttle control at idle. A good, precise servo will allow you to have a very predictable idle that won't die at really bad times (like during a spin).

Power: this is probably the most important factor. Read through Peter Goldsmith's article on the IMAC website about trimming a plane for precision. ANY control surface blowback will make trimming a pain because the servo won't be at its intended position as airspeed changes.

I think the best advice I ever got was to spend my money on good servos. You will probably never know just how much you apprecaite them until you've tried the alternative, inexpensive route. Save your money on graphics and spend it on servos if you want to get serious about precision flying!!

Ken