Well, since Titan orbits Saturn, you can't "sling" around Saturn, since it's the primary body of Titan.
Not precisely true. You won't gain velocity relative to a solar orbit, but you can change your velocity vector. It's not a traditional gravity assist type of slingshot, but it is still technically a slingshot.
In this specific instance, I was flying the "Mars Bound Veep" scenario, which I believe came with the DGIII (this was in '05 or thereabouts). When the scenario starts, Titan has entered the anti-sunward bound portion of its orbit. A direct flight from Titan to Mars would have necessitated eliminating the outward velocity imparted by Titans orbit. Using Saturn to re-direct that velocity inward saved substantial Delta-V. TransX didn't seem to consider that a slingshot, and while I could come close to the course I wanted it was not very efficient - only saving me about 20% of the fuel used compared to a direct transfer (fuel savings mostly due to Oberth effect)
IMFD was able to set up a slingshot course, but it was even further from what I wanted. It assumed that the only burn would be the Titan ejection burn, and only used Saturn for re-direction. It also planned a TEj months in the future. I simply used two OrbitMFD's (one referenced to Saturn, one Titan) to plan an ejection along a Saturn Retrograde direction, then once clear of Titan, used a Hohman transfer to place myself at the Saturn Pe called for in IMFD. When I got close I performed the TMI burn and saved fuel due to the Oberth effect. End result was a Mars arrival 2 months sooner than TransX could get me there, and an additional 25% fuel savings.
Granted, someone more knowelegeable may have been able to get better results from TransX. The version that I had at that time lacked much support for a retrograde slingshot, even the 3.0 manual says only that they are "problematic", and that future versions would hopefully have that functionality. That may have improved since Agentgonzo took over, but if so it doesn't appear to be documented.
TransX's main advantage is it's abilitiy to plan dozens of stages, allowing a much more complex flightplan than IMFD. A "Grand Tour", for instance, would be difficult if not impossible with IMFD. IMFD can't do everything TransX does, but what it can do it does easier and more accurately.