I had trouble with most of the calendar based sign-out tools since everything was by period and not by a specific time.
1. Easy to use
2. Uses period instead of times
3. Web-based, hosted externally
4. No login required
1. Expensive $199/yr ~ it is designed for EDU so that is the schools cost.
2. Doesn't snyc with GAFE so users need to be manually imported.
3. No login required. I have not run into any issues yet, but technically malicious teachers or students could signup for others or delete others labs. However it does keep a log which I think could show if someone had it and then was deleted.