When driving down a normal interstate you won't see any difference. However, it's during those more extreme situations when IFS makes a difference. Here's a few examples:
1) Potholes - When you encounter a pothole the shock is transmitted acrossd the entire axle. One wheels dips into the hole and changes the angle of the axle, then bounces back up out of the hole. This, plus the additional unsprung weight of the heavy front axle mean that you will feel a bigger jolt when driving through potholes or over a bump.
2) Driveway approaches - Face it, you don't enter a driveway dead on square. Your coach is always at an angle as you turn into the driveweay. This means that first one wheel negotiates the ramp, then the other. The end result is that the coach rocks from one side to the other. With IFS the wheels are free to independantly lift so you don't get as much rocking action as with a solid axle.
3) Speed bumps - Same as above. If the speed bumps are at an angle you get the rocking effext with a solid axle but no so much with the IFS.
4) Front tire blowouts - These can be deadly and will throw your coach into the ditch in short order if you can't keep it on the road. Then things really get ugly. With IFS you have a much better chance of maintaining control than with a solid front axle. A solid axle requires a course in Charles Atlas course in weight lifting to prepare your arms for that task.
However, in 90% of your driving you won't find much difference between a good, well designed solid front axle (like the Spartan) and IFS.