I went into a T-Mobile store to take a look at the Sidekick over the weekend. I didn't expect much from the sales staff, especially after hearing some stories, but they at least knew about the Sidekick and were taking orders. I was hoping they'd have a functioning demo model so I could check to see if a few web pages worked in the unit's browser, but they don't do demo models at T-Mobile stores. They had a fake Sidekick shell to demo the size and feel of the thing, and I have to say it felt great. It's only a little wider than my cell phone. But it didn't have a working screen, so I couldn't say much beyond that.
While the staff appeared willing to sell me one, they knew almost nothing about it (though tried to fake their way through a couple of answers)-- not their fault, they haven't been trained on the thing yet. They were even trying to talk me out of it, saying they heard a "rumor" about a future model having a color screen, an empty sentiment considering it's obvious a future model will have a color screen. They might be turning away the early majority (me) until service improves; the early adopter customer experience isn't for the faint of heart. They also confirmed that the "upgrade path" for existing T-Mobile customers is confusing and potentially unpleasant, and we're not eligible for the $200 post-rebate unit price. In any case, while it sounds like this first generation model fulfills expectations and would be fun to have around, I'm sold on the idea to wait until the next model (or a few well-received major software updates) before considering it again.
In lieu of an SDK, Sidekick users are setting up web apps to perform simple functions missing from the device. A simple server-side web calculator and a simple networked chess game should tide over current owners for the time being.
Good luck, Danger and T-Mobile! Keep it cheap, open the SDK, and build a reputation for quality service at a low price point (despite your rocky start), and you'll own the new hybrid cellular phone/Internet market, even with your proprietary operating system.