I find that my systems respond sluggishly when I am making use of them AND boinc is running. I know that I am not taxing my systems with document writing and web browsing, but my 'user experienced' is diminished while boinc runs.
It would be great if the boinc application could be prioritized lower when the user is logged in AND the "while computer is in use" option is checked. Optionally, the "use at most X% of CPU time" option could be used in coordination with the "while computer is in use" option.
Overall what I'm shooting for is full use (by boinc) of my system when I am away, and partial use while I am logged in and doing work.