I was one of the first Sidekick customers, long before the MS buyout. It was an amazing email/texting device, far easier and more comfortable to use than a Blackberry. Plus the web browsing was decent, and the UI was simple and elegant. All in all, a really cool phone.
Unfortunately, the first generation of hardware was super flaky. I had five of them die on me in a year, often failing within weeks. Since there wasn't any PC sync tool, the only thing that saved my bacon was the server-side data model. When a phone died, I'd get a replacement, drop in the SIM card and all my stuff would be there same as always.
Back in the day, there was no data sync with your PC. I think they eventually released Outlook syncronization, but most consumers don't use Exchange/Outlook. I would guarantee that a huge number of Sidekick owners (most? all?) have never had any of their phone data anywhere but the Danger servers.
To make matters worse, the Sidekick becomes kind of a brick when the servers are down. I can see a lot of people resetting their device because it's suddenly become unresponsive, just expecting their data to magically come back from the server.
This is an unmitigated disaster.