On May 15th, once Diablo 3 launches, I could try and get it from a local store, or I could reserve a copy of the game in advance; then bring the physical copy home and install it on my computer. I could instead buy it right from blizzard via digital copy, with the risk that I wouldn't be able to get onto the servers due to everyone else trying to download the game at the same time. What is the better choice?
Diablo – Pros and Cons of Buying Diablo 3 hard copy vs digital copy
diablo-3
Best Answer
You can pre-download Diablo 3 once you've purchased it online. As @dlanod mentioned, you can generally pre-download Blizzard games.
However, to more directly address the question you have proposed you need to remember/be-aware that Blizzards downloader is a Peer to Peer downloading scheme. They use small feeders to populate different sections of the game to different people and then all of those people are offered up as hosts to everyone else to get those parts.
So the situation is not really a few hundred thousand people all hitting one or two servers for the game, but a few hundred thous and people all getting a small piece pretty quickly from the server like normal web browsing or MMO traffic or whatever, and then sharing that with everyone else as long as they are downloading.
There should not really be any release day concerns with access the server to download the client to your system.