Well, discluding my first Blue version (which lost all it's data thanks to me being a little kid who let the big kids mess with my game via a game shark) where I started with Charmander, I always start with the water type on my first time around, and Diamond was no exception. If a third game in the D/P set ever comes out, I'm planning on going for Turtwig.
As for what I think of the starters, I love everything about the Turtwig line. Absolutely nothing wrong with it in my book. As far as Chimchar's line goes, it's okay... but also a Torchip ripoff. A good starter, but I would have liked something... new. :D;;; The Piplup line is alright, though Prinplup has gotta be one of the ugliest things I've ever seen. Still, I love it.
Uh er sorry if that offends anyone, just telling it like it is :(