Sorry no one has seen fit to post any comments so far. I'll just chip in with a few comments; First off, I'm not a grafitti artist either, but from what I have seen the letters you've used are a little too legible compared to proper 'NYC Subway grafitti' text. You definitely seem to be going in the right direction, but you've got to take a bit more of a chance with the design I'd say. To be honest, there are some great grafitti type fonts out there that you may be better off using as a starting point, then adapting it as you go along.
Also, in case you haven't done so already, just google for some ideas on grafitti. It always helps to see what is missing that's not immediately obvious, even when you know it's not quite right.
Good luck
JH


