Since there hasn't been much response directly about the N550, I'll relate an experience with the N750. My uncle bought one because he felt (and I agree) that the differences/upgrades more than justified the price increase over the 550.
Long story short, it took several weeks and 3 returned units before the 4th one worked correctly. Every unit had a different problem. One was the power button, one was the contrast knob (I think), the 3rd problem I don't recall. After all that, they tried to send him a version that didn't include USB for custom reticles, saying it was a "patent issue". They finally sent one with it after he asked why it was ok that the first 3 had it.
I've spoken to one experienced 550 owner who was quite shocked to hear about the issues. I don't know if it's a fluke, a bad run, early model QC, I can't say. May not apply to the 550s at all but I'd be leery, personally.