Resource borrowing is a common underlying approach in grid computing and thin-client computing. In both cases, external processes borrow resources that would otherwise be delivered to the interactive processes of end-users, creating contention that slows these processes and decreases the comfort of the end-users. How resource borrowing and user comfort are related is not well understood and thus resource borrowing tends to be extremely conservative. To address this lack of understanding, we have developed a sophisticated distributed application for directly measuring user comfort with the borrowing of CPU time, memory space, and disk bandwidth. Using this tool, we have conducted a controlled user study with qualitative and quantitative results that are of direct interest to the designers of grid and thin-client systems. We have found that resource borrowing can be quite aggressive without creating user discomfort, particularly in the case of memory and disk. We also describe an on-going Internet-wide study using our tool.