ITN usually refers to ballistic captures/throws via Weak Stability Boundaries (WSBs) to/from L1 and L2 necks. I don't like Ernestopheles' answer. Conflating this with swing by gravity assists muddies the waters.
Shane Ross and friends are credited with inventing the term Interplanetary Superhighway. From Shane Ross' page:
Impractical for Interplanetary Transfers Due to Long Transfer Time:
Due to the long time needed to achieve the low energy transfers
between planets, the Interplanetary Superhighway is impractical for
transfers such as from Earth to Mars at present.
SEL1 Sun Earth Lagrange 1
SEL2 Sun Earth Lagrange 2
SML1 Sun Mars Lagrange 1
SML2 Sun Mars Lagrange 2
My convention is to put the initial for central body first and orbiting body second.
The Sun Earth L1 and L2 points are about 1.5 million kilometers from earth. This is only 1% of an A.U.
Payloads nudged from SEL1 or SEL2 would follow paths like this:
The ellipse from SEL2 has a 1.07 A.U. aphelion. This doesn't come close to the ~1.52 A.U. aphelion needed to reach Mars.
An ITN defender might reply "But repeated gravity assists from earth might boost aphelion."
The 1.01 x 1.07 A.U. ellipse has a period of of 1.063 years. It's synodic period with regard to earth is 16.87 years. (think of two runners running almost the same speed -- it takes the slightly faster runner a long time to lap the other)
Does this mean a nice gravity assist every 17 years or so? Not quite. For a good gravity assist, it would need to pass by earth during perihelion. A flyby during perihelion would only occur once every 8th fly by. So gravity assists every 136 years or so.
And as the earth comes up from behind the payload, it pulls it backward. Then after it passes, it is pulling the payload forward. So usually the net effect of a fly by is nearly zip. It would take millennia to get an aphelion up to 1.52 A.U.
A key quantity in 3 body mechanics is (mass orbiting body)/(mass orbiting body + mass central body) often denoted μ
Some μ numbers for various orbiting bodies:
Earth Moon .012
Sun Jupiter .00095
Sun Saturn .00029
Sun Neptune .000052
Jupiter Ganymede .000078
Jupiter Europa 000025
Sun Earth .00000304
Can comets ride WSBs from one gas giant to another? Sure. Could a Jupiter orbiter ride WSBs between Galilean moons? Sure.
But a WSB from Earth to Mars? That is a bridge too far.
I go into more detail at my blog post Potholes on the Interplanetary Superhighway.
Notice the earth moon has a hefty μ. Using WSBs for travel about the earth moon neighborhood is much more interesting. I write about this at my blog post EML2
So in response to "How useful is the Interplanetary Transport Network" I'd reply "Not very".