This thing ROCKS! It's smaller, lighter, more accurate, and last but not least, it's A LOT easier to use. On the 305, I had issues with the charge contacts on back corroding. They have eliminated this possibility by sinking these holes into the watch and having a clip like charger. It's really cool, but hard to explain. This is a definite improvement.
Before I get into this, if you are new to the Garmin 405 check out the training videos on the Garmin Website. These are very helpful. Most definitely the coolest thing about it is the training setups you can do. This will teach you many of the nuances of the 405, and they're adding things weekly to help us learn how to use this wonderful tool. If you're in to this like I am, it's also very easy to just play around and figure it out. , I use the interval mode a lot. Here's one example. When I go out for Yasso 800's, I click on the Menu, Click Training, Workouts, Interval, then you choose if you want this workout to be gauged by time or distance (for Yasso's I click distance), then choose the distance (.50 miles), then rest type, I choose distance (.25), then choose how many reps (8), then it asks about a warmup and a cooldown (I choose yes). This takes all of about 1 minute to set up.
I have used the Virtual training partner, but I haven't fallen in love with this yet. It's kind of hard for me to tell if I'm ahead or behind, I have to look really close to be able to tell. I can see this being a nice feature, especially in a 5k or 10k race. I have gotten over the sensitivity of the bezel, it takes a little getting used to, but once you do, it's really not an issue.
Now, the complaints, my biggest complaint is that the ANT agent is still not compatible with Mac's. They say this is coming in the Fall, so I'm going to hold them to it. Luckily, I have access to a PC where I can download my information. I do have issue with the watch not giving detailed Heart Rate information. I can only get my Average Heart Rate by downloading my information to the PC, then I have to look at the Heart Rate statistics. I've notice when I'm sweating a lot, if sweat drips on the bezel it gets a little crazy beepy. Hopefully this doesn't cause problems when I really start dripping this summer.
Overall, to me, this watch is a major upgrade. In defense of the 305, I never really used the training features, but I think that's because it was too complicated. This is a very nice tool to help in training no matter what the distance.