Took the plunge, and swapped out the steel build plate for a boro
About the only thing you want in terms of features is 220mm diameter (largest standard size that fits in the stock plate footprint), and you want the edges of the glass to be finished (because they'd be dangerously sharp otherwise). This one I ordered from Amazon has nice ground edges, but I suspect there's little or no difference between sellers on this. Everyone seems to be selling the same glass pieces from China.
you have a few choices on mounts. this 3-part, clip-on mount is what I went with, as it looks great. The downside is that it's three parts to print, and it takes a while. You end up with something that looks stock, however.
You other choice is something more utilitarian. Not as good-looking, and you have to be careful about printing at the edges, as the build plate is now offset from where the software thinks it is, but it's functional.
Upsides tot he mod: the build plate is less likely to cause adhesion problems, as the steel plate had quite a bit of thermal mass. You could mitigate that by warming the plate up before printing. I'd run it under hot water, especially when cleaning it for the next run - before I figured it out, I was washing it in cold water, which would cause adhesion problems every time.
Doesn't appear to affect the calibration process one bit. It finds it with the same light tap it performs on the steel.
It's also a very nominal modification (though pretty close to the cost of a second steel plate). I paid $16 for the glass, and a handful of hours to print the clips (the plastic costs are negligible).
Downsides: It's glass. You need to be more careful handling the build plate, and if you drop it, you'll be standing in (or holding) glass fragments. The steel plate would merely crush your foot, but it wouldn't shatter. =P