Works fine, but there are two things to check - battery life and waterproofness. Most car units are built with the assumption that they'll usually be plugged into 12 VDC power and therefore only need the internal battery to operate for a short time. And most are not waterproof so you may need to keep them covered (ziplocks work ok) if you might find yourself riding in the rain. You can usually solve the battery life problem by adding an external battery pack.

Also check to see if the unit you have in mind lets you specify that you're using it on a bicycle (the Garmins you mention above do) so when you push the 'go home' button it doesn't direct you to the nearest freeway onramp.