I have a 315 as well and have given it to my brother to use. There was a firmware upgrade at one time because it lost its fix if you were moving too slow, like when you are trying to narrow down the position of a cache. I used to have the problem of moving to a cache fairly quickly then slow down to get close. I'd all of a sudden find my GPS jump 30-60'.
If you did the "honey bee" dance (waving your arms around like you where fighting off bees), it moved the GPS fast enough to retake a position where you are standing. The firmware update fixed that. I can't remember what the update was or version number, sorry.
I now have a Meridian Green and really like it. I have MapSend Streets and it is great. I won't go back to the 315. Having the street info for the area where a cache is works really well. I've also used it on trips to find where I'm going (driving) as well as to help find my way back to the campground (driving) when its dark. It gets satellites much faster and works overall a lot better.
I now want to get an Explorist 500 with a color screen.