I owned a unibody white Macbook for five months. While there was nothing fundamentally wrong with the computer itself (I liked it quite a lot, actually), I really grew to hate the white polycarbonate shell over time. It's NOT made of the same sort of durable plastics that you'd find on a Thinkpad or even a 2001 vintage Sony Vaio (I have one; it looks almost new to this day).
For instance:
Every surface on the shell except the area immediately surrounding the keyboard has a glossy finish. These surfaces scratch and scuff very easily; the nylon security strap in my old laptop bag (I eventually replaced it with a felt-lined messenger bag) really did a number on it. You only have to look at it from an angle or under direct lighting to see what automotive detailing hobbyists would term "swirl marks." I also noticed spots on the finish starting to rub dull on the wrist rests next to the track pad after just a couple months (and this was a secondary computer that saw very little use when I was at home near my desktop).
The keys show dirt and seem to smooth out and/or discolor from use/skin oils. I was obsessive about washing my hands with soap before using the computer (especially after eating), but the end result was still the same--it just took longer to develop.
Unlike the MBP (with rounded feet), the entire underside of the Macbook chassis is rubberized. This surface stains and dirties very easily; moreover, I've read reports where the entire rubber surface eventually begins to separate from the baseplate. I didn't own mine long enough to see that happen, and I'm not sure it necessarily would have (doesn't seem to be a ubiquitous problem), but the fact remains.
The cracking seems to be less an issue than it was with the pre-unibody versions, but it's important to remember that this chassis has only been around for 16 months.
Long story short? I developed a serious case of buyer's remorse, sold it this past week, and ordered a new (2011) MBP.
Now, these are relatively superficial problems, and many (most?) people won't care or even notice. I'm just trying to point out that physical durability is not a legitimate advantage for the plastic MB.