I have only used VISE IT, they are phenomenal.  They seem to lock in good, they don't move around with your hand in the ball too much, and they don't fall apart.  Ive had a couple turbo guys complain they dont last and they can feel them move in the ball.  On the other hand I witnessed a VISE IT snap and it wasn't pretty. 

I don't personally use them, but have heard good things about the Grp-Loc System.

You will get people liking all of them.  The question you should be asking is "what are teh best interchangeable thumb systems for people who grab the ball a lot."  My experience has been that they all work good for people who don't grab the ball.  With Grip-Loc, when a bowler repeatedly grabs the ball it wears notch in the locking L mechanism on the slug and then even though the inner is secure, the inner will move slightly in your backswing making you think it is coming out.  With Turbo which is what I personally use, I have experienced some tabs breaking, but more commonly wearing out and therefore not wanting to lock as securely as when the inner was brand new.  I haven't heard of any problems with any of the interchangeable thumb systems from people who DON'T grab the ball.

I won't tell you which is the best as I have no first hand knowledge working with some of them, but I can tell you I have installed over 500 ITs into customer's balls, and have not had a single one fail. I have had 2 slugs seperate from the bottom piece (slug-it). Ironically, they did it at around the same time, and I attributed it to bad glue. I began installing the IT system 4 years ago, and they definitely have held up over the years. IF THEY ARE INSTALLED PROPERLY, they will work great. There are quite a few little quirks and tricks about them that make them a lot simpler to work with. Feel free to PM me if you want any additional info.