The ease in which to use, ability for one person to do multiple jobs at once (e.g., entering results into lab notebooks) without having to switch between programs or computers; it runs well even with older hardware/software configurations.
I dislike how difficult it was to find an answer online when something wasn't working right after we installed it - there were so many errors being posted about problems people had while using this program! It's very easy to learn if you have any experience doing manual data entry but it does require some training time as most staff are not used to computerized systems like this. We've been able to reduce our turnaround times by over 50% since implementing winlims.