I got it just to have something as a testbed for Windows 10 and use as a 1st gen iPod Touch replacement. Comparing the two, weight feels about the same which surprised me. The phone feels great in the hand - the shape is comfortable and the size is just right for comfortable use. My main device is a Lumia 1020 so the 550 doesn't feel thick at all. Actually it's the opposite - it looks reasonably thin to me. Though of course it's noticeably thicker than the old iPod.
I have the white variant and surprisingly I really like the white glossy finish - hides fingerprints well (can't say the same about the screen glass) and doesn't feel 'slimy', like my matte 1020 can sometimes feel after not being cleaned for a while. Though of course it never feels as solidly built and dense as my 1020 (and it's not meant to be). Yes, the phone feels hollow because there's a gap between the battery and the cover and there's some give in the cover because of that. Strangely, the 630 I bought for my mom as a first smartphone last year feels more solid and better built just because it doesn't have that give in the back cover. But for 120 euros I can't complain. The 640 is more expensive here and that made the 550 a good deal.
What really surprised me on this phone is the screen. I think it's rather good - it can get really bright and pixel density is adequate (better than my 1020s just because it doesn't have a pentile matrix which can show jaggies in some instances). Also, screen's color accuracy is decent. Especially for the budget. Much better than my 1020s which has a greenish/yellowish tint that is not correctable through settings (and that always astonished me how a photography oriented phone has such an inaccurate screen).
All in all, hardware wise it's a nice little package for the money. What lets it down is software.