Firstly, only reply if you have experience using API V3 (i can speculate myself!)
I had a little go with V3 and it looked great but would love to hear from someone who's given it a bit of use before I start working with it and deploy it on a live site.
I'm only looking to do very basic things:
It all looks very easy with v3: http://www.svennerberg.com/2009/06/google-maps-api-3-the-basics/
is it stable enough?
I have been using V3 for about a month, on a replacement site currently under test. I have experienced no problems with it (at least, no problems that I couldn't blame on my own incompetence...). The stability seems fine, mind you I am not using the more "esoteric" features of the api; just displaying a map, showing markers, opening infowindows.
I've been using V3 for a little while now, and have found that while it is indeed quite lightweight, the learning curve is quite steep.
I've had to develop a number of functions and methods myself, by hand, in order to create a desired effect.
I'd say it should depend solely on how in-depth you're going to get. If you want an incredibly thorough, feature-rich map which makes the most of the API, I'd probably go with v2.
If you just need a simple map with markers, simple overlays, some path-finding, and all the basic stuff you can find currently on Google's reference pages, by all means go with the more lightweight v3.
I went with 3 because I wanted to focus on mobility and usability. While most of the things I required were already in v3 ready to use, I've had to create a rather complicated Javascript algorithm to create my own custom paths based on the shortest path, which the user can interact with(a close example would be Dijkstra's Algorithm).
The problem with v3 that I'm finding is that most things I've had to do myself. It's simply not been out long enough for there to be a huge listing of features and tutorials and such.