I use the Vise-IT system in all of my bowling balls and really enjoy the same feel from ball to ball, with the versatility that comes from the IT system.  I am someone who doesn't keep bowling balls for more than one season (normally), so interchangeable slugs works great for me.  But, if you are someone who keeps bowling balls for a longer period of time, I have seen some people have issues with resurfacing bowling balls multiple times and the fit of the interchangeable thumbs.  What I mean is, if you have a ball that is resurfaced many times, and you always leave the slug out, eventually the slug is going to sit up from the surface of the ball, and could cause grip issues where it is not flush against the ball.  On the other hand, if you always leave the slug in a ball to get resurfaced, you will wear down the slug to the point where if you put it in another, freshly drilled ball, it will sink down into the ball and not sit flush, also causing some gripping and/or pinching issues with your thumb.

Like I said, I love the versatility of having the same feel in each ball from the IT system, and since I change bowling balls each season I don't have any issues like I listed above. There are some ways to get around those issues, but I am just trying to give you all the positives, negatives, and stories I have seen from other people using the interchangebale system. I hope that helps.