205/305 are big for a watch, but small for a gps. i have run about 10,000 miles in my 305 and dont notice it all.
just changed to the 310xt as the 305 finally crapped out.
most useful feature by far for me is being able to upload my runs and see where i went on googlemaps. (i use a mac program called rubictrack). if you run a lot on trails or when travelling this can be very very useful for discovering new runs.
the other features ive used a few times.
- "take me home". the watch can caluclate a reverse route for you from where you are. only really needed if you get hopelessly lost.
- uploading waypoints. useful if going into the wilderness. you upload a known waypoint and it can show you it on the map, and tell you which direction/distance etc.
obviously all the pace, distance functions are useful too, but you dont really need to go with a costly gps to get that.
305 is probably the bargain watch to get. now the 310 is out, you can get 305 pretty cheap. the 205 is the the same as the 305 but doesnt have the ability to communicate with hr strap, bike sensor & footpod.