I used a Garmin Etrex Vista HCx for several weeks touring in Europe last year.
It was a great help, and had a reasonable selection of POI on the map.
I've also used the TomTom app on an iPhone, and it works well until the battery dies.
Some points to consider:
- iPhone apps chew through the battery, you will only get a few hours.
- you probably need a mapping GPS for what you are doing, the non-mapping ones are only useful for pre-planned routes, or for recoring your track
- the auto-routing is not 100% reliable. It will sometimes take you on a 20km diversion to avoid a short section of highway, or a bridge that it takes a particular dislike to.
- a GPS that takes AA batteries rather than a built in battery means that you can buy some spares if the battery goes flat. Alternative is somethin like the PowerMonkey, but that's another box to carry (and maybe lose)
- an outdoor or bicycle unit is much more water resistant than a car model, and usually has longer battery life
- thoroughly test the bike mount on some rough roads to make sure it is secure and doesn't rattle.
(a dab of Shoe Goo does a good job of suppressing rattles)
- I always lash the lanyard from the GPS to the stem, but then I am a bit paranoid about losing gear
- I found it worthwhile to have a simple Cateye computer as well, just to have a backup for speed and distance
I've had good results from my Garmin, the TomTom may be a bit better at navigating, but I think it depends more on the quality of the maps than anything else. The newer Garmins can take raster maps (e.g. detailed topo maps) which is a real advantage.
Have a look at a Garmin Edge 800, or a Dakota 20. They work out around the same price, usually, when they have equivalent extras.
The Garmin Oregon and the Satmap have bigger screens, but that means that mounting them on the bike is a bit more difficult.