Has anyone used these instead of T-pieces for their cooling system I see they are expensive but just wondered if anyones had much success with them? Do they leak etc?

Wouldn't be my first choice; looks guaranteed to store up problems for the future...
You can get a Tee-piece from CBS for the same price.

I use one of those at the highest point in my system (as it is on an elbow unfortunately).

Seems to work fine for me, only leaked a little when I had a coolent pressure issue (dropped cylinder liner so I was pushing stupid PSi through the system) but apart from that is fine!

Mark