Depending on the purpose you want these for, these are an amazing deal! They are made of a quality glass, and capped with plastic. The plastic pieces are attached to the glass solidly, no risk of sand leaking out.
Our porposes are pretty general, but I still tested our timers. Glass can be a hard medium to get to do exactly what you want, so some variation can be expected, but heres how ours went:
1min Timer: ran about 55sec
3min Timer: spot on
5min Timer: Ran about 15 seconds over
10min Timer: Stopped 35 seconds early
We wanted the 1m timer for teeth brushing, so I just have to inform my kid to count to 5 after it ends before finishing. We felt the 5m was within the margin of error. More than half a minute off leaves the 10m timer almost useless for a timing purpose.