I've owned a 3595 for longer than any other phone- about 3 years now. I got it on eBay for $17.
The phone I've owned the longest as my primary phone that I actually use is my Blackberry Pearl. I bought it right when it came out in September. Although it has a few flaws, I'm really happy with it overall and haven't found a better compact smartphone.
I also owned a Sidekick 2 for two separate periods time totaling 10 months. It was a really cool device, and the UI is absolutely amazing, but it's just too limited. Specifically, I needed a phone that I could tether to my macbook, and the sidekick doesn't offer that feature. Furthermore, there are many other issues with the sidekick, including the sidekick 3. The screen is horrible, it's bulky, no mms, terrible camera, etc.