While I would not buy a rollerskate from CHMB's sales department (after the experience when my smart was delivered there in 2008) I have found their service department (especially Service Writer TOM G) to be very good.
As is obvious, they are primarily a Mercedes dealer (no smarts on the showroom floor - no literature, etc) and they charge Mercedes rates. I had used them for a number of years prior for work on various Mercedes both in warranty and out. All work, including some diagnostics on the smart was done well and quickly. You do have to ask for what you want and know what is not really necessary.
I will admit, for some major out-of-warranty work on a Mercedes lately I went to an independent and saved about 40% because it was basic, but major work (final cost over $2k). Snyderman's Gulf at 2nd and Vine (560 N. 2nd) was who I used.