Below is a mcmap rendering of my world with biome support, note that only the area enclosed in red has been generated after the halloween update.
Now as for my feelings on this, I do think biomes are too small in general, or maybe just too random in size, there are "rainforests" I can jog across in a few seconds while to my consternation my main base seems to be centered in a large tundra.
Pre-halloween biomes of course take no account for tree density or in the case of deserts, sand, but I could fix that manually with an axe if I felt like it.
The largest improvement I would like to see from Notch in the future is a more sensible biome distribution. Moisture tends to eminate from bodies of water in warmer climates and is dropped on the ground as it climbs in elevation. Dry areas are formed in the lee of mountain slopes or inland away from warm water.
While it may not be practical to create a "global" climate in a square world, some sense of consistency in the distribution of temperature and moisture is needed.