A few things to keep in mind:

All your data is stored on their servers, so read those terms and conditions carefully.

The platform isn't officially 'open' in the way that Symbian is, so third-party software (esp. free software) might be hard to come by.

From the photos I've seen, it looks cheap and plasticky, however they could have been prototypes - keep build quality in mind when checking it out.

And size is quite an important factor; if you're going to get the sidekick, get a small phone as well so you have something to use when you don't want to lug the sidekick around.