my favorite thing is that you can "pop" out the connector (not easy, but doable) and create female/female and male/male track pieces.
Our original set is incomplete and being able to rig the pieces of the IKEA set to fit the other set has restored my faith in my engineering abilities

We also have the extra train which has turned out to be kind of superfluous, unless your child is old enough to make both a freight and passenger train.

ETA: Ikea sells both a train set and a separate train, we have both.