I too came from a 9000 I did a review a few weeks ago, but in short here are specific answers to your questions.
Keyboard - The keyboard on the 9000 was by far the best that RIM ever put on a device. You do get used to the keyboard on the Torch, but I still fat finger more than I did with the Bold 9000.
Performance (OS) - I noticed only occasional lagging with the original OS. Upgrading to .246 addressed it. The update went smoothly.
Memory - No issues.
Usability - I will vouch that if you're a hard core BB user, there is an adjustment period with the Torch. You have many different ways to navigate. Swipe, slide, touch, use the track pad or keyboard. After a week it felt second nature.
Overall I too am glad I held on to it. The touch screen is very good and I love the flexibility.
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