Thoughts on the Gravitystorm Transit Layer
First off, it looks great. The transit features pop out and I'd love to see his other styles running in Maply as well. Andy is putting a lot of data into his tiles and for mobile devices I might suggest backing off on that a bit. Adding another level might work too.
There's room for improvement, but given that these weren't designed for mobile devices (!) it's amazing how well they work. The styles come through nice and sharp. For a customer I might tweak the line widths and such, but overall it's lovely.
How About Maply?
Of course, I do these things to test WhirlyGlobe-Maply. The vector tiles were delivered as an MBTiles file rather than being paged from a server. Now the toolkit supports that. I found a few color related bugs, a few parsing problems, and some features in the Mapnik XML I wasn't supporting correctly. Those are fixed, but I do see a few others things that could be done.
Overall I'm pleased with how Maply renders these tiles. You could build a credible map app with the Gravitystorm vector tiles on iOS using Maply. Perhaps you'd like to give it a try. :-)
Mapnik vector tile support is in WhirlyGlobe-Maply 2.3.