I just finished reinstalling my head unit, using a second dash kit and I wanted to share what I learned:
Unfortunately, I made the mistake of buying my first installation kit from an ebay vendor known as autoware302: Negative/Neutral Feedback received by autoware302
He shipped the item through the US Mail in a stolen, inside-out FedEx Tyvek document envelope with no bubble wrap, packing peanuts, etc. It arrived with mounting ears bent and broken. He ignores e-mails, has no phone number, and, as you can see from above, has a long history of dissatisfied customers. Avoid at all costs!
There are three smart fortwo dash kits made by Scosche, Metra, and American International. My comments appear below.
Scosche (ST2430B)
This is the kit I ended up using. Because I was installing a single-DIN Alpine iDA-X100 (highly recommended), the available storage pocket below was welcome. The pocket that I received was bowed in at the top and bottom and I ended up using a heat gun to work the bowing out -- probably removed from the mold before fully cooling. Annoying, but not a deal killer. There are also four small holes (two top, two bottom) in the pocket at the back (see above to see one). Not sure why they are there, but it makes the pocket unsuitable for tiny things like paperclips unless you cover the holes as I did. The finish was a bit smoother than I would have chosen and the black was not a perfect match for the factory dash, but it's quite acceptable and would not bother most people at all. This kit sells for $15-$20.
Metra (99-8714)
I ruled this kit out as it does not support ISO mounting (screws through the side of the radio). Instead, it requires a DIN sleeve into which the radio slides. DIN sleeve mounting tends to hold the radio less securely, both from a crime standpoint and resistance to vibration. I don't want a radio that can be popped in under 15 seconds by a thief. This kit sells for $30-$35, making it the most expensive.
American International (SM-K108)
This is the first kit that I bought. Very good texture and color match on the bezel. Divider between top and bottom units very thin. Divider bowed out about 1/8". Single DIN filler panel snaps into slots, but tabs needed to be thinned a bit to go into the slots. That there is no storage pocket makes this appear very unfinished and like something is missing below the head unit (radio). If you have gauges or other things to mount in the panel (such as AC power outlets, USB connectors, Scangauge II), it could work well if glued into position, but I would not trust it to stay in place with just the two little tabs. Overall, a very clever and sturdy design that interlocks without screws (other than the factory screws that hold it all in place). If I were installing a double DIN unit, this is the kit I would choose.
Note: Image is inverted relative to normal mounting.
If you are looking to purchase a kit via the Internet, I'd go for a respected vendor like Sonic Electronix, Installer.com, or millionbuy.com.
I hope that this proves to be useful.