I have the ITs in all of my balls. For me the biggest advantage in using them is
the ease in switching thumb slugs from one ball to the next. This is a big advantage,
especially when I bowl PBA senior regionals. No more spending time taping a zillion thumbholes. From a purely functional standpoint however, they do not provide me any advantage
over a ball that is conventionally drilled and slugged. Both must still be drilled in accordance to my particular specs. I also do not vary the pitches in my slugs because doing so affects my release since the ball was originally fitted to match my particular span-to-pitch-measurements.   

In addition, I personally have had no success switching to oval (thumbs). They were very comfortable, but in my hand they were way too sensitive to the slightest variation
in their (angular) placement . I could not get a consistent release with ovals, so I have long since switched back to the conventional (round holes).

Again, it's a matter of personal preference.