I'm probably one of the few who use a PDA for navigation on a motorcycle. It has it's advantages and disadvantages over an all-in-one (AIO). Aside from typical PDA functionality, I think you've touched on the former: A wide variety of choice in the kind of software you can use. I currently have 3 different navigation programs, each with it's own strengths and weaknesses. I also have a couple of 'moving map' programs where I can load topos etc., and I have a professional mapping application that I use for work related stuff.
Another advantage is that you can pick and choose hardware: My current PDA is an Airis (same as a MIO and available only through a single U.S. distributor) that has a built-in GPS. It has served me quite well since I bought it in January. Before that was an HP Ipaq combined w/ a bluetooth GPS receiver. Fine for in-car, but too complicated on a motorcycle, and the Ipaq was simply a piece of crap.
I keep my Airis in a Ram-mount 'Aqua-box' (large) that happens to be a perfect fit. Keeps it good and dry, but visibility is a real problem, especially on a sunny day. This is the biggest drawback to my setup that I have found.
As for navigation software, I have:
- OnCourse Navigator 5: which is a re-branded version of Europe's Navigon software. This is no longer available as a PDA application in the U.S. Navigon will be entering the U.S. market directly, but I think only as an AIO device.
- OnCourse Navigator 6: which is a re-branded version of Igo, used by MIO in their AIO units. This was just released a few months ago, and has some great navigation algorithms, but I find the interface frustratingly complex and missing some really key features like custom POI loading and a limited route planning tool. This is what I have been using most however.
- CoPilot Live 6 from ALK: By all rights, this should be the application of choice. It has some great features and just about all the bells and whistles one could ask for, except for one really big problem: the map data is hopelessly flawed and almost useless in my area! I have been (im)patiently waiting for an upgrade to CP7 which was supposed to have been released this month (May 07). If they at least improve the maps, it should be the way to go.
Right now, TomTom is probably the best bet in the PDA navigation market unless ALK can get their act together. I would have gone with them to begin with except there were some issues with the TomTom software I didn't like at the time, which I think have since been corrected.
Unfortunately, this is a small market segment which is being led primarily by GPS-enabled smart-phones. Still, I think it is perfect for in-car use, and can be good on a motorcycle if properly protected. Not perfect, and certainly not as easy and handy as the Zumo, but doable.