I'm not typically an early gadget adopter, but I was so into the whole concept of the Sidekick I got one of the early ones. The good old black and white, nice fat ones. Surprisingly, I wasn't disappointed.
I travel a lot, so having access to email while on the road was a great feature. The Sidekick is always connected, so no dial-up and waiting for connections. It's just there. The data on the Sidekick is automatically backed to the Web. Create a new note, it's automatically on the T-mobile site. Take a picture, it's automatically on the Web. I can take notes on the Sidekick, and when I'm on the computer, I can simply access them through a Web browser. The device also has IM and Web browsing built in. Too many times I have read news headlines while stuck in traffic. Once I actually booked a ticket on Southwest while on the way to the airport via the Sidekick browser, which was silly but cool (I could've much more easily just dialed Southwest and booked it that way, but I'd have lost the double frequent flyer credit).
The cons for this thing are construction and reliability - I believe I had mine fixed or replaced three times, and still it was never quite right. It's also a bit too bulky. It's terrible as a phone - awkward to hold, and just generally too big. I used it strictly for data.
And then came: Sidekick II
My trusty black and white Sidekick eventually became crippled through lots of use and abuse, so I get the new Sidekick II. I really like this device. It has all of the benefits and features of the original, but feels a lot more solid, is more compact, and is usable as a phone. The speakerphone sounds quite good. T-mobile is also finally offering an outlook synchronization capability for a one time $10 fee, which is well worth it for me.
I purchased this from Amazon for $175 list, with $200 rebates, making it -$25 in theory. $50 of the rebate is from T-Mobile, and requires you to be on a plan of $39 or above. The problem is, there's also a $20 per month data access fee, costing you at least $50 per month total (if you take a $30/month voice plan). The alternative is to go with an unlimited data only plan, which costs you $30 per month. I decided to forego the $50 rebate and switched to the $30 unlimited data plan as soon as I got the phone. I have a separate phone, so for me this made sense.
In practice this worked out to $25 after rebate, plus $35 activation fee, plus $30/month.
UPDATE: Here's Adam Bosworth saying Sidekick II rocks.
Grab yourself a T-Mobile Sidekick II Phone if you have a need for email, IM, or Web browsing from anywhere, or just like cool gadgets.