I got the 305 for Christmas in 2009 and absolutely love it. It has a few more bells and whistles than the 205, including a heart rate monitor, which some people use as a training tool over pace and distance (I don't, but I have triathlete friends who do). I've never had an issue with battery life and it seems pretty durable.
I use the 305 whenever I run outside and have used it in about three or four 5Ks and have found it to be extremely accurate. I find that when I train I become fixated on the pace readout, which can be good or bad. The distance and pace indicators are brutally accurate. There is no fudging your statistics with the Garmin. I like the fact that the displays are fairly large and clear as my 40+ eyeballs don't see things up close like they used to.
My only criticisms are that the 305 is kind of bulky and once the readout froze and I had to do a soft reset. Also, once in awhile it takes several minutes to link up with a satellite, but these are minor quibbles. I noticed the 405 at Costco last week and it looks really nice, more of a watch look than the 305, but also more expensive.