I like to keep as few things open as possible, I view 'system monitoring' gadgets as being a tad pointless: modern computers should not need monitoring by design.

If there's 'widget-sized' information I need like unread email count, the current weather, or upcoming calendar appointments I've got my docked iPhone for that.

In a way, Vista's 'sidebar' concept makes more sense from a usability perspective, but to me it's all just a gimmick. The sidebar gadgets are just a really poor version of OS X's Dashboard (which does the job properly): they're not the prettiest, there's barely any animation effects, they don't scale, etc.