Samsung's build quality is one of the best. Look at the quality issues that phones have. Samsung devices usually have the least of them.
Like Casey said, their plastic might
feel less premium than aluminum or other materials but it's by no means of inferior quality. I doubt it's cheaper than aluminum too. Samsung Wave smartphones that have aluminum unibodies are actually cheaper than similar Galaxy smartphones.
I don't own a Samsung device now, I own a Sony phone, I used to own one HTC phone, a Motorola phone and a few Nokias and I've never had a device more durable than my 2 ex smartphones from Samsung, even though one of them was plasticky.
Aluminum covers usually suffer from chipping issues, are heavier and look
worse after a drop or two, so you actually take sacrifices for a more premium look.
Not to mention that Samsung pack the best internals in their phones in any price range because they can, because they're manufacturing 90% of their phones by themselves. So they're not fooling anyone by any means - they simply offer the best products.
It's just sexy, man. I'm not going to try and explain it to you if you don't see it.
To me it looks very average. I don't know anyone with a Lumia phone so I googled for images and checked review scores and some people say that it looks very good, while others say that its design is rather poor but the phone doesn't win design awards by any means so I'd say it's a matter of preference here.