Not much more to say what the first reviewer hasnt already mentioned.
I used a Macbook Pro until now (which had - after < 2 years of use - enough bugs for 3 laptops - fan out of control, cd/dvd drive broken, etc.), and I dont regret switching to the W520 (I own a W520 for a month now). I am doing a lot of Matlab image processing on my laptop, and the performance as well as screen resolution / quality is great (upon purchase, Lenovo lets you choose between 3 different screens with different maximum resolutions - i chose the highest resolution - 1920 X 1080; also they let you choose between 2 different gpus). One unique thing to mention is that you have full control over which graphics processor (integrated vs. Nvidia gpu) your laptop uses when, and for which application separately - i think this is kind of new and unique.
The only thing that bothered me initially was Lenovo's trackpad, which doesnt come close to the comfort of my Macbook pad. Normal cursor control is ok, but e.g. two-finger scrolling is almost impossible. After my initial disappointment, I got really hooked to using the red lenovo track point. It takes one to two days to get used to it, but once you have the hang of it, it's much more convenient for cursor control, scrolling (instead of 2 - finger scrolling, one uses the middle button above the trackpad), etc. than the track pad. So - no complaints there. And if you want to do e.g. technical drawing, you may want to use a mouse in any case...
One thing more that took me a bit of patience to figure out was the fact that - by default - the operating system shuts down and 'unmounts' the physical cd drive when not in use. This is a good feature as it helps saving energy. However, it interferes with the installation and usage of virtual drives (e.g. Daemon tools). Anyway, I figured out that you can easily disable this CD drive - auto-shutdown option. So - if you have the money - stop looking. This is your machine.
