The nuvi is designed for in-car use, where you have a nice steady supply of electricity from the cigarette lighter socket and protection from rain. Since you have neither on a bike (unless you have a clever solar panel or only take short rides, and only ride in fair weather) the nuvi starts to fall down there.

Someone else mentioned the Dakota, which is a pretty nice piece of kit. The touchscreen is fairly easy to get used to but if you've got fat fingers (or gloved fingers) you might find it a struggle. A friend who has one always takes a pen to use as a stylus with it (with the nib retracted!). The Oregon series also have a touch screen but personally I found it almost impossible to read in sunlight. I struggled a little with the Dakota but not to anything like the same extent.

I like Garmin's GPSMap 60CSx, which is sturdy and doesn't have a touch screen so you can read it in just about any light. It's waterproof (when mine gets muddy I wash it under the tap) and a decent set of AA batteries will last all day and then some. It's an old unit now (replaced by the 62 series) so you can get them fairly cheaply (probably about $200 for a new one, less for a used one). For maps you can either buy Garmin's maps (which can be expensive) or, depending on where you live, you may be able to find free opensource maps for the areas of interest.

Just one thing, if you get a 60CSx make sure it's the CSx and not the CS. The CS doesn't take memory cards and, for today's purposes, has very little internal memory.