Background: I've had sidekicks since the first version was released with the monochrome screen. I've lost count of the number I've actually owned. The heritage of the Sidekick LX is obvious -it has the same set of buttons shared by every generation. Physically, it's thinner and lighter than the SK3. Once again, it has the removable battery and a micro-sd card that's accessed underneath the back cover. The screen flips open as usual, and the keyboard is made of a single membrane. Nothing sticky should enter the keys (unlike previous generations).
The UI is pretty much the same, but looks even better with the new Sharp Aquos based display. It does feature a few changes, but they're very congruous with the OS. It's got the glossy updated OS feel - sort of like Mac OSX or Windows Vista. It appears that applications are aware of the screen state now - you can't type in an address search until you open the screen in the phone application.
The T-mobile desktop looks unchanged, and my previously downloaded outlook sync application worked perfectly. My biggest complaint has to be the desktop interface - it really hasn't been updated in years. Like the SK3, the phone will play mp3s, but I haven't tested it yet. Music lovers rejoyce - the LX features a standard 1/8 inch headphone jack. You should be able to use your favorite cans with this one.
I haven't tried out the dev tools for this latest generation. I was underwhelmed by the overuse of XML in the past. Getting a Developer key was a snap as usual. Danger's path to delivering applications has proven to be a bit retarded from an end user perspective, but if you want to develop for these things, check out their developer area. They have free simulators to go along with the free SDK, so you don't even need your own device at this point.
I've noticed one real problem with the LX. The side buttons are entirely too subtle. It's usually not a problem, but during a call I simply can't locate the volume buttons by feel. It's not a deal breaker, but it is a bit lame.
Despite the lack of say... 8GB for music and no video playback, I keep coming back to the sidekicks. Why? They're reliable, email notifications are consistant and don't take over the OS (like windows mobile), IM works great, the physical form factor is just right and unlimited data, despite being slow, is the best deal around at $20/month. I used to hate the lack of tethering (sharing internet to your laptop), but after having a few phones that did it, realized that tethering just sucks. You don't want to do it anyway. Get an EV-DO card if you need net access - the sidekick is a great stand alone device. The new LX is just beautiful. The blue is more nearly black, and it's certain to get a few raised eyebrows when you whip it out.