I use the onboard ethernet pretty constantly, mainly because I find the wireless to be finicky and hard to troubleshoot when you're running a headless setup. I can always plug in an ethernet cable, though...
i connect my htpc via a Wi-Fi USB dongle as well. it runs both Linux and windows 7. and despite the os and drivers one model always hang after a day or so. another one, different model from same brand is online for months at a time.
it's mostly badly designed/tested hardware.
You can get it working for a while, but it's nowhere near the reliability of standard domestic WiFi.
I've built various sensor/hardware projects and tried various dongles, and I've always had to switch back to a cabled connection.