I'd argue the T40 was great in an era where PC laptops were shit to begin with. Yeah, it was the best plastic one, but to me it's no outstanding model. The Macbook Pros back then were probably more amazing in construction. I'm writing this on my ThinkPad for work. It's good in the sense that it's better than my crappy Dell at home, but I'd take my MacBook Pro 2008 over this. Surely any unibody MacBook Pro feels even better today. The only complaint for me is the sharp edges, but that's not so much a build quality issue as it is a design issue.
Moreover, it's a very industrial old look, and in today's world having that washed out TN panel makes it a very disappointing product when facing manufacturers who are finally focusing on the screen.
I do not know that Nokia slider, but I have owned and used a few. The Nokia N80 and N95 are prime examples of slider phones. None of those are considered good build quality. They wobble and creak easily. They're good phones overall, but none of them feel really solid in your hand.