Also, it shows you how much delta-v your ship has as you build it, and as you fly it. Delta-v is effectively a much more useful measure of "fuel" than your actual fuel.
I'm gonna give the same advice I give everyone who is told about mechjeb, learn to pilot manually before using mechjeb. I suggest using it once certain parts become routine (launch, rendezvous, precise powered landing)
Yes I agree totally, it's part of the fun. What's great is if you're in career or science mode you'll still need to work up to the parts needed to use MJ. You'll have to do the first orbits and rendezvous yourself.