Is success 1 million units first week sales? Nexus One did 20k units, Droid 250k, iPhone 3GS 1.6M (from Flurry). Apple sold 75M iPhone and iPod touch devices in 2009. About 20M netbooks were sold in 2009. Apple sold about 6M Apple TVs in 2009, and that's "just a hobby" for them.
Maybe initial sales don't matter and the iPad will be judged by its adoption 3 years from now, by the copycats it inspires or by the way changes the personal computer industry. After all, Apple reinvented the desktop PC industry once, but ultimately lost it to Microsoft. I doubt they will make the same mistake again.
Update:Apple just announced 2 million units sold in the first 60 days. It took over two years for Apple to sell its first 2 million iPods. The original iPhone took about four months to reach the 2 million mark. I'd call the iPad's launch a success.