How reliable are the see through blocks?

I'm gonna be upgrading my WC setup sometime in the next few months, and I was wondering how reliable are the see through (I'm assuming it's acrylic) plastic tops for blocks? They have a tendancy to leak more or would I be pretty safe buying/using one.

Acrylic tops with the threading added to them for barbs will typically only last 2 years until stress fractures start to form. Although the problem is less pronounced when straight threads are used (G1/4 instead of NPT 1/4) the potential is still there. If you want a block that will last a long while, go with one that has a delrin, copper, or brass top.