I have glass tiles in a shower. They don't show water spots because they are a matte finish but shiny ones probably would. (My chrome fixtures do spot up.) I'd considered using these in the kitchen so I tested them for the usual. Basically, it's glass. You know, tough stuff. Can you stain a beer bottle? Nope. Glass is awesomely tough. Get a sample and test it like you would a countertop. You might be able to etch it, but if it's just glass, you won't be able to stain it.

Grout on the other hand, that's the potential problem. :)

We had glass subways in our last house and though we did get spots every once in a while, it was not a regular occurrence, and it cleaned right up with no problem.

I has glass mosaic tiles in my tub/shower in the cottage. Absolutely no problems. With glossy glass, the larger your expanse of glass, so the bigger the tile surface, the more likely you are to notice spots.

I have matte glass tiles as the backsplash in my kitchen. They are easy to clean and don't show grease very much.

what I notice is when new, glass doesn't spot (like Fori's example of a beer bottle) but over time it shows. If you left the bottle out to air dry every day, spots show. So thinking I should lean toward porcelain or matt glass tiles.