What has been your experience with the personal helium balloon tanks?

We picked up a balloon time one yesterday and I'm disappointed! I blew up the balloons about 2 hrs pre party and by the end of the night the balloons were falling down. I got a Mylar number balloon and it didn't even float?

I got it thinking I'll use it for dd2's 1st birthday in 4 months. Now I'm wondering if the tank will leak out before then?