Accuracy of Garmin 305 elevation??

it really depends.

vertical error is exponentially worse then horizontal error.  If your satellites have good spread and are located near the horizon, it can be pretty good.

I've never had any real issues.  Most of the error will either minimal or glaring (sudden 1000' jumps) and the smoothing algorithms in Garmin's software usually filter that out.  If you sign up for motionbased (it's free), and upload your data, you can check for yourself.  They have a high resolution terrain database built in, so you can either use the recorded GPS elevation, or have motionbased lay the 2d data on their terrain field and get a 'true' picture.