For those that have trouble with small displays on basic computers, try the Cateye Padrone. It has a very large, clear speedometer.

The garmin 700 and up have very modifiable displays- you can make a single field/tile large if desired (though tbh I've never attempted to use the entire display as one single large field). Same with the 500, though it still has a fairly small display. A lot of money to pay for a big display though.

Cell phone apps are convenient (and becoming more so with all the apps and bluetooth gadgets), but I don't like the idea of carrying my expensive phone on my handlebars, nor needing to worry about battery life (I already use my phone a lot and need to watch the battery enough as it is). I've seen people kill their phones using them on the bike in the summer heat, too.